Solution Studies of Ru2(O2CR)4n+ Complexes (n = 0, 1; O2CR

Inorg. Chem. 1996, 35, 3643-3658

3643

Solution Studies of Ru2(O2CR)4n+ Complexes (n ) 0, 1; O2CR ) Octanoate, Crotonate, Dimethylacrylate, Benzoate, p-Toluate) and Solid-State Structures of Ru2(O2C-p-tolyl)4(THF)2, [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]-, and Ru2(O2C-p-tolyl)4(CH3CN)2: Investigations of the Axial Ligation of the Ru2 Core Malcolm H. Chisholm,*,† George Christou,† Kirsten Folting,† John C. Huffman,† Chris A. James,‡ John A. Samuels,§ Jodi L. Wesemann,| and William H. Woodruff‡ Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405-4001, and CST-4, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed July 7, 1995X

Ru2(O2C(CH2)6CH3)4 (1a) is soluble in both coordinating (THF, CH3OH, CH3CN) and noncoordinating solvents (benzene, toluene, cyclohexane, CH2Cl2), allowing its solution properties to be investigated by 1H and 13C NMR spectroscopy, UV/visible spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry. In noncoordinating solvents, 1a exists as an oligomer, presumably by way of axial intermolecular -(- -[Ru2]- -O- -)n- interactions. 1H NMR studies of 1a and [Ru (O C(CH ) CH ) ]+[X]-([1a]+[X]-), where X ) Cl, BF , or O C(CH ) CH , 2 2 2 6 3 4 4 2 2 6 3 indicate that both dipolar and contact mechanisms contribute to the paramagnetic shifts of the protons. Resonances for axial and equatorial ligands are shifted upfield and downfield, respectively, by a dipolar mechanism. Aromatic ligands in the axial sites, e.g. pyridine and pyrazine, experience an enhanced upfield shift by direct π-delocalization. Comparison of the 1H NMR signals for M2(O2CR)4 compounds where M ) Ru and O2CR ) benzoate, toluate, butyrate, crotonate, and dimethylacrylate with those where M ) Mo indicates that the equatorial carboxylate ligands in the diruthenium species also experience π-contact shifts. Variable-temperature studies and calculated estimates of dipolar shifts (using structural parameters taken from solid-state structures) indicate a significant zero-field splitting contribution to the dipolar shift. The arrangements of the toluate rings in Ru2(O2C-p-tolyl)4(THF)2, Ru2(O2C-p-tolyl)4(CH3CN)2, and [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]- deviate by 15(1), 2.3(2), and 7.3°, respectively, from alignment with the Ru-Ru axis. The Ru-Ru distances for the two neutral and the cationic complexes are 2.27(1) Å, i.e. not significantly affected by the nature of the axial ligand (THF versus CH3CN) or by charge n+ (n ) 0, 1). The cell parameters for Ru2(O2C-p-tolyl)4(THF)2‚2THF at -154 °C are a ) 10.730(5) Å, b ) 12.335(6) Å, c ) 9.193(4) Å, R ) 105.15(2)°, β ) 109.35(2)°, γ ) 77.98(2)°, Z ) 2 (asymmetric unit is RuC24H30O6), dcalcd ) 1.559 g/cm3, and space group P1h. The cell parameters for Ru2(O2C-p-tolyl)4(CH3CN)2‚3CH3CN at -169 °C are a ) 27.058(3) Å, b ) 10.049(1) Å, c ) 17.956(2) Å, β ) 120.89(1)°, Z ) 4, dcalcd ) 1.465 g/cm3, and space group C2/c. The cell parameters for [Ru2(O2C-p-tolyl)4(THF)2]+[BF4]- at -172 °C are a ) 13.056(4) Å, b ) 21.358(6) Å, c ) 9.199(2) Å, β ) 111.28(1)°, Z ) 2, dcalcd ) 1.350 g/cm3, and space group C2/m.

Introduction Dimetal tetracarboxylates, M2(O2CR)4, represent an important class of transition metal complexes. For M ) Cr,1 Mo,2 W,3 Ru,4-6 Rh,7 and Cu,8 a paddle-wheel or lantern-like structure is adopted in the solid state and each metal is further coordinated along the M-M axis either by a neutral ligand or via intermolecular interactions with its neighbors9 as shown in I. †

Indiana University. Los Alamos National Laboratory. § Current address: Watkins-Johnson Co., 440 Kings Village Road, Scotts Valley, CA 95066-4081. | Current address: Department of Chemistry, Harvey Mudd College, Claremont, CA 91711. X Abstract published in AdVance ACS Abstracts, April 1, 1996. (1) Cotton, F. A.; Rice, C. E.; Rice, G. W. J. Am. Chem. Soc. 1977, 99, 4704. (2) (a) Cotton, F. A.; Daniels, L. M.; Kibala, P. A.; Matusz, M.; Roth, W. J.; Schwotzer, W.; Wang, W.; Zhong, B. Inorg. Chim. Acta 1994, 215, 9. (b) Martin, D. S.; Hueng, H.-W. Inorg. Chem. 1990, 29, 3674. (c) Cotton, F. A.; Mester, Z. C.; Webb, T. R. Acta Crystallogr. 1974, B30, 2768. (3) (a) Baxter, D. V.; Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Putilina, E. F.; Tagg, S. L.; Wesemann, J. L.; Zwanziger, J. W.; Darrington, F. D. J. Am. Chem. Soc. 1994, 116, 4551. (b) Chisholm, M. H.; Chiu, H. T.; Huffman, J. C. Polyhedron 1984, 3, 759. ‡

S0020-1669(95)00860-3 CCC: $12.00

Within this series of compounds, the M-M interactions vary from M-M quadruple bonds (M ) Cr, Mo, W) to relatively weak antiferromagnetic coupling (M ) Cu). For M2(O2CR)4 complexes with 4-fold D4h symmetry, the d-orbital splitting (4) (a) Bennett, M. J.; Caulton, K. G.; Cotton, F. A. Inorg. Chem. 1969, 8, 1. (b) Bino, A.; Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1979, 18, 2599. (c) Martin, D. S.; Newman, R. A.; Vlasnik, L. M. Inorg. Chem. 1980, 19, 3404. (d) Drew, M. G. B.; Higgins, P.; McCann, G. M. J. Chem. Soc., Chem. Commun. 1987, 1385. (e) Higgins, P.; McCann, G. M. J. Chem. Soc., Dalton Trans. 1988, 661. (f) Cotton, F. A.; Matusz, M.; Zhong, B. Inorg. Chem. 1988, 27, 4368. (g) McCann, M.; Carvill, A.; Guinan, P.; Higgins, P.; Campbell, J.; Ryan, H.; Walsh, M. Polyhedron 1991, 10, 2273. (h) McCann, M.; Carvill, A.; Cardin, C.; Convery, M. Polyhedron 1993, 12, 1163. (i) Cotton, F. A.; Kim, Y.; Ren, T. Polyhedron 1993, 12, 607.

© 1996 American Chemical Society

3644 Inorganic Chemistry, Vol. 35, No. 12, 1996 pattern yields the well-known arrangement of M-M molecular orbitals10 shown in II. This simple pictorial description is

pedagogically pleasing but not necessarily accurate since it ignores the mixing of other orbital interactions. For example, for Mo2(O2CR)4 and W2(O2CR)4 compounds, the M-M σ orbital is not unique because the ndz2 orbital mixes with both the filled (core) npz orbital and the valence (n+1)s orbital. Moreover, the M-M π and δ orbitals can mix with the oxygen pπ orbitals of the bridging carboxylate ligands. The energy separation among σ, π, δ, δ*, π*, and σ* orbitals will also vary with M-M distance, and this in turn will be influenced by the metal, its oxidation state, and the nature of the axial ligands. For M ) Cr, the Cr-Cr distance spans a range of ca. 0.4 Å as a function of axial ligation,11 and the search for an axially free Cr2(O2CR)4 compound is an interesting story in itself. Certain workers have argued that Cr2(O2CR)4 compounds do not contain M-M quadruple bonds on the basis of calculations,12 while others have argued against such M-M bonding on the basis of the kinetic lability of the Cr-Cr bond toward rupture in the presence of strongly donating ligands.13 The carboxylates of ruthenium,4-6 Ru2(O2CR)4n+ where n ) 0 or 1, also have an interesting history with respect to assignment of their electronic structure. The Ru24+ complexes have two unpaired electrons5b,5d,14 while those with Ru25+ cores have three.4i,15,16 Although the ground state electronic configurations (5) (a) Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1987, 2723. (b) Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1985, 2321. (c) Lindsay, A. J.; Tooze, P. R.; Motevalli, M.; Hursthouse, M. B.; Wilkinson, G. J. Chem. Soc., Chem. Commun. 1984, 1383. (d) Cotton, F. A.; Miskowski, V. M.; Zhong, B. J. Am. Chem. Soc. 1989, 111, 6177. (6) (a) Das, B. K.; Chakravarty, A. R. Polyhedron 1991, 10, 491. (b) Spohn, M.; Stra¨hle, J.; Hiller, W. Z. Naturforsch. 1986, 41B, 541. (7) (a) Cotton, F. A.; Shiu, K.-B. ReV. Chim. Miner. 1986, 23, 14. (b) Reference 2a. (8) (a) Campbell, G. C.; Haw, J. F. Inorg. Chem. 1988, 27, 3706. (b) Lomer, T. R.; Perera, K. Acta Crystallogr. 1974, B30, 2912. (c) Lomer, T. R.; Perera, K. Acta Crystallogr. 1974, B30, 2913. (9) Mo2(O2CCPh3)4‚3CH2Cl22a was shown recently by single-crystal X-ray diffraction to have no axial interactions. (10) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 2nd ed.; Clarendon Press: Oxford, U.K., 1993; p 18. (11) Cotton, F. A.; Feng, X.; Kibala, P. A.; Matusz, M. J. Am. Chem. Soc. 1988, 110, 2807 and references therein. (12) Hall, M. B. Polyhedron 1987, 6, 670 and references therein. (13) Hao, S.; Edema, J. H. H.; Gambarotta, S.; Bensimon, C. Inorg. Chem. 1992, 31, 2676 and references therein. (14) (a) Marchon, J.-C.; Maldivi, P.; Giroud-Godquin, A.-M.; Guillon, D.; Skoulios, A.; Strommen, D. P. Phil. Trans. R. Soc. London A 1990, 330, 109. (b) Maldivi, P.; Giroud-Godquin, A.-M.; Marchon, J.-C.; Guillon, D.; Skoulios, A. Chem. Phys. Lett. 1989, 157, 552. (15) (a) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28, 2285. (b) Mukaida, M.; Nomura, T.; Ishimori, T. Bull. Chem. Soc. Jpn. 1972 , 45, 2143. (c) Cotton, F. A.; Pedersen, E. Inorg. Chem. 1975, 14, 388.

Chisholm et al. for these carboxylates are now generally accepted to be σ2π4δ2δ*2π*2 5d,17,18 and σ2π4δ2δ*1π*2,19,20 as shown in III, the

ordering of the δ* and π* orbitals is not as intuitively obvious as the d-orbital splitting pattern shown in II would suggest. Metal-ligand interactions influence their relative energies, and it is apparent that the orbitals must be very close for Ru2(O2CR)4n+ where n ) 1 and are likely to be close for n ) 0. The usefulness of the simple M-M σπδδ*π*σ* orbital description becomes more limited for these open-shell configurations because of spin-spin and spin-orbit interactions. Although the Ru2 core has been examined with a variety of bridging ligands in addition to carboxylates to ascertain the effect on electronic structure,20 no studies have involved systematic variation of the axial ligands of Ru2(O2CR)4L2, nor has anyone determined the ground state of the unligated Ru2(O2CR)4. Even the best ab initio calculations17a that were performed on Ru2(O2CH)4 were carried out on a Ru2(O2C)4 core with metric parameters taken from Ru2(O2CCH3)4(H2O)2. The effect of the axial water molecules of ligation was not considered in terms of its influence on the M-M distance. In this paper and in the one to follow,21 we have examined the solution behavior of the hydrocarbon-soluble octanoates of diruthenium with respect to their binding of axial ligands by the use of paramagnetic NMR spectroscopy and other techniques. We have gained insight into the nature of Ru2-ligand interactions and information pertinent to the molecular design of extended one-dimensional polymers incorporating Ru44+ centers. Results and Discussion The Octanoates. (a) 1H NMR Studies.22 The 1H NMR spectra of Ru2(O2C(CH2)6CH3)4, 1a, in noncoordinating sol(16) Telser, J.; Drago, R. S. Inorg. Chem. 1984, 23, 3114. (17) (a) Quelch, G. E.; Hillier, I. H.; Guest, M. F. J. Chem. Soc., Dalton Trans. 1990, 3075. (b) Clark, D. L.; Green, J. C.; Redfern, C. M. J. Chem. Soc., Dalton Trans. 1989, 1037. (c) Clark, D. L.; Green, J. C.; Redfern, C. M.; Quelch, G. E.; Hillier, I. H.; Guest, M. F. Chem. Phys. Lett. 1989, 154, 326. (18) Bonnet, L.; Cukiernik, F. D.; Maldivi, P.; Giroud-Godquin, A.-M.; Marchon, J.-C.; Ibn-Elhaj, M.; Guillon, D.; Skoulios, A. Chem. Mater. 1994, 6, 31. (19) Norman, J. G., Jr.; Renzoni, G. E.; Case, D. A. J. Am. Chem. Soc. 1979, 101, 5256. (20) (a) Cotton, F. A.; Ren, T.; Eglin, J. L. Inorg. Chem. 1991, 30, 2552. (b) Cotton, F. A.; Ren, T.; Eglin, J. L. J. Am. Chem. Soc. 1990, 112, 3439. (c) Cotton, F. A.; Feng, X. Inorg. Chem. 1989, 28, 1180. (d) Cotton, F. A.; Matusz, M. J. Am. Chem. Soc. 1988, 110, 5761. (e) Chakravarty, A. R.; Cotton, F. A.; Tocher, D. A.; Tocher, J. H. Polyhedron 1985, 4, 1475. (21) Wesemann, J. L.; Chisholm, M. H. Submitted. (22) Historically, paramagnetic NMR data were reported so that the sign of the chemical shift corresponded to the field (upfield shifts were positive). In this report, the current standard convention is used (upfield shifts are negative). The equations stated here correspond to this convention.

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3645

Figure 1. 1H NMR spectra of 1a in (a) toluene-d8 and (b) THF-d8 (asterisk ) solvent).

Figure 2. 1H NMR spectra of [1a]+[BF4]- in (a) toluene-d8 and (b) THF-d8 (asterisk ) solvent).

vents, such as benzene-d6, toluene-d8, dichloromethane-d2, and cyclohexane-d12, consist of relatively broad and unresolved signals, as shown in Figure 1a. In contrast, the 1H NMR spectra for 1a in coordinating solvents, such as THF-d8, methanol-d4, acetonitrile-d3, and acetone-d6, show sharp and well-resolved signals (Figure 1b), more typical of a diamagnetic sample except that the resonances are shifted to lower field. Several explanations were considered to account for the marked difference of the NMR spectral features. (1) Since compound 1a is readily oxidized, we were concerned that some oxidation of the Ru24+ core might have occurred. This, however, was ruled out on two grounds. First, use of Evans’ method on samples of 1a in both coordinating (THF) and noncoordinating (toluene) solvents indicated the presence of two unpaired electrons (µeff ≈ 2.8 µB), as expected for a Ru24+ center. The Ru25+ center has three unpaired electrons. Second, the 1H NMR spectra of cationic complexes [1a]+[X]- in both coordinating and noncoordinating solvents are similar to each other and are readily distinguished from those of 1a. As shown in Figure 2, the 1H NMR signals for [1a]+[X]in both types of solvents are fairly well-resolved and span a larger range (-50 to +20 ppm) than those for 1a. (2) The possibility that the spectra like those shown in Figure 1a result from the axial ligation of benzene or toluene, which has been seen for certain Cr2-11,23 and Rh2-containing24 tetracarboxylates, is ruled out by the fact that the same spectral features were observed in dichloromethane-d2 and cyclohexaned12. While dichloromethane is known to bind weakly to Cr2(OC(CH3)NR)4 complexes,25 complexation of cyclohexane is unprecedented in these systems. (3) A change in the electronic ground state could have occurred. For Ru2(O2CR)4 complexes where the axial site is occupied with an oxygen atom from either water or neighboring

carboxylates magnetic susceptibility measurements show the ground state to be δ*2π*2.5d,18 In the absence of a σ-donor ligand, the energy of the most stable state arising from electronic configuration δ*1π*3 might be lower than that from δ*2π*2, or at least close enough to be populated to a significant extent at room temperature. Both states are S ) 1 states and therefore may not be distinguished by Evans’ method noted above. The electronic structures of other Ru2(bridge)4 compounds have been studied,20 and of these, the triazenido complexes are diamagnetic, having the σ2π4δ2π*4 ground state and an increased RuRu distance.20d We would expect that an increased occupation of the Ru2 π* orbital in Ru2(O2CR)4 would have a marked effect on the Ru-Ru distance. While we were not able to determine the structure of 1a, we were able to study the effect of various coordinating and noncoordinating solvents on the ν(Ru-Ru) stretching frequency by resonance Raman spectroscopy. (See later.) There is, in general, a relationship between M-M bond length and the value of ν(M-M). However, we observed very little change in ν(Ru-Ru) as a function of solvent and donor ligand and conclude that the further occupation of the π* orbital and a change in electronic ground state are most unlikely. (4) Finally, we considered the possibility that 1a was in fact an oligomer in noncoordinating solvents, existing as a piece of the chainlike structure typically found for unligated M2(O2CR)4 compounds in the solid state. (See pictorial description shown in I.) Cryoscopic molecular weight determinations were carried out in benzene, and they supported this view. Indeed at ca. 5 × 10-4 M, compound 1a showed an average degree of association corresponding to a trimer of dinuclear units, i.e. [Ru2(O2C(CH2)6CH3)4]3. The 1H NMR spectra were recorded on more concentrated solutions, which most likely involved an even greater degree of oligomerization. These spectra also showed temperature dependence, the broad signals sharpening with increasing temperature, indicative of dynamic exchange. For comparison, we examined the 1H NMR spectra of Rh2(O2C(CH2)6CH3)4, which is a diamagnetic molecule. It too showed somewhat broad and ill-resolved 1H NMR signals in

(23) Cotton, F. A.; Daniels, L. M.; Kibala, P. A. Inorg. Chem. 1992, 31, 1865. (24) Moodley, K. G.; Chisholm, M. H. Unpublished results. (25) Cotton, F. A.; Ilsley, W. H.; Kaim, W. J. Am. Chem. Soc. 1980, 102, 3475.

3646 Inorganic Chemistry, Vol. 35, No. 12, 1996 Table 1.

1H

Chisholm et al.

NMR Assignments Based on T1 Values for 1a and [1a]+[X][1a]+[O2C(CH2)6CH3]+ b 1aa

CH2(2) CH2(3) CH2(4) CH2(5) CH2(6) CH2(7) CH3(8) a

[1a]+[BF4]- a

equatorial

axial

ppm

T1 (ms)

ppm

T1 (ms)

ppm

T1 (ms)

ppm

T1 (ms)

3.2 7.4 7.6 4.3 3.4 2.6 1.6

198 ( 1 275 ( 5 344 ( 3 702 ( 5 958 ( 7 1420 ( 20 1870 ( 10

-42.4 16.5 11.8 6.3 4.7 3.6 2.0

5.5 ( 0.7 10.2 ( 0.2 14.8 ( 0.3 45.7 ( 0.2 73.9 ( 0.3 122.0 ( 1.0 223.0 ( 1.0

-42 11.6 7.3 3.9 3.2 2.6 1.9

11.1 ( 0.3 18.7 ( 0.1 31.7 ( 0.8 76.0 ( 1.0 123.0 ( 1.0 173.0 ( 1.0 236.0 ( 1.0

-27 -24 -11.6 -4.5 -1.4 0.3 0.8

7(1 19 ( 1 45 ( 2 87 ( 5 154 ( 5 251 ( 6

in THF-d8. b In toluene-d8.

toluene-d8, which sharpened upon heating. Cryoscopy also indicated that the Rh2 complex was not monomeric in benzene, and an average n for association was estimated to be 2. For both M ) Ru and Rh, it is likely that the degree of association decreases with increasing temperature and the rate of formation and rupture of oligomers by -(- -[M2]- -O- -)n- interactions increases. Our conclusion is that the dramatic difference in the appearance of the 1H NMR spectra of 1a in coordinating and noncoordinating solvents is due to the strong desire of Ru2(O2CR)4 molecules to have their axial sites occupied. In coordinating solvents, 1a(L)2 molecules are present, where L represents an axial ligand provided by the solvent. In noncoordinating solvents, oligomers [1a]n form via axial -(- -[M2]- -O- -)ninteractions. UV/visible, resonance Raman, and cyclic voltammetric data for 1a in various solvents are consistent with this explanation. (b) Assignment of the 1H NMR Signals of 1a and [1a]+[X]-. Owing to the paramagnetic nature of 1a and [1a]+[X]-, their NMR resonances are shifted from the values seen for the analogous diamagnetic dimolybdenum tetraoctanoate Mo2(O2C(CH2)6CH3)4. The shift caused by the unpaired electrons is referred to as the isotropic shift (eq 1)and results

δobserved ) δdiamagnetic + δisotropic

(1)

δisotropic ) δdipolar + δcontact

(2)

from the distribution of unpaired spin density via dipolar (through-space) and contact (through-bond) mechanisms (eq 2).26-28 Assignment of the 1H NMR signals and comparison of their isotropic shifts indicate that both dipolar and contact mechanisms occur within the diruthenium tetracarboxylates. Since the 1H NMR signals for 1a in THF-d8 fell in a narrow range, 2-D NMR experiments were used to make assignments. The COSY spectrum is shown in Figure 3. The initial assignment of CH3(8) was made on the basis of the relative intensity, simple triplet pattern, and chemical shift of the signal at 1.6 ppm. This signal was not significantly different from that of Rh2(O2C(CH2)6CH3)4, as would be expected for protons far away from paramagnetic centers. Cross-peaks correlated the CH2 protons. The basic pattern was an increasing shift in the downfield direction, the protons closer to the metals being more affected. This trend diminished for CH2(3), whose signal did not shift as far downfield as might be expected, and reversed for CH2(2), whose signal was shifted back in the upfield direction. T1 measurements (Table 1) confirmed the COSY assignments, the protons with the shortest T1 values being closest (26) Satterlee, J. D. Concepts Magn. Reson. 1990, 2, 69. (27) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders Co.: Philadephia, 1977; Chapter 12. (28) Drago, R. S.; Zink, J. I.; Richman, R. M.; Perry, W. D. J. Chem. Educ. 1974, 51, 371, 464.

Figure 3. 2-D COSY spectrum for 1a in THF-d8.

to the metal-metal bond. T1 measurements were also used to assign 1H NMR resonances for [1a]+[X]- systems (Figures 2 and 4, Table 1), which possess a third unpaired electron and display similar but exaggerated chemical shift patterns.29,30 Thus, both dipolar and contact mechanisms appear to contribute to the isotropic shifts of equatorial ligands, one causing downfield and the other causing upfield shifts. More detailed examinations (see below) show that the downfield shifts for the equatorial carboxylate protons in Ru2(O2CR)4n+ are due to dipolar (through-space) mechanisms and that the upfield shifts, most obvious for the CH2(2) protons, may be attributed to contact (through-bond) mechanisms. The axial ligands of diruthenium tetracarboxyaltes seem to experience both dipolar and contact mechanisms as well. As seen in the 1H NMR spectrum of [1a]+[O2C(CH2)6CH3](Figure 4), the axially-coordinated carboxylate gives rise to a (29) The chemical shift of CH2(2) is dependent on the axial ligand. For [1a]+[BF4]-, an upfield signal at -41 ppm was seen in methanol-d4. For [1a]+[Cl]+, in methanol-d4, which does not completely replace axially-coordinated Cl-,30 two signals were seen upfield at -39 and -41 ppm. Addition of NaCl increased the concentration of axiallycoordinated Cl-, indicated by the increased intensity of the signal at -39 ppm. (30) Drago, R. S.; Cosmano, R.; Telser, J. Inorg. Chem. 1984, 23, 4514.

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3647 Table 2. [1a]+

1H

NMR Resonances (ppm)a for Axial Ligands of 1a and

L pyrazine 2,6-dimethylpyrazine pyridine 4-picoline THF Figure 4. 1H NMR spectra of [1a]+[O2C(CH2)6CH3]- in benzene-d6 (asterisk ) solvent).

distinct set of signals. Compared to the signals for the equatorial ligands, these signals show a reversed pattern, with signals appearing increasingly upfield for protons closer to the metal center. The CH2(2) resonance was not shifted as far upfield as might be expected, once again suggesting the occurrence of competing contact mechanisms. Studies on various bis-ligated 1a and [1a]+[X]- species (see below) show that the upfield shifts for the axial ligands of Ru2(O2CR)4n+ are due to dipolar mechanisms (and π-contact mechanisms with aromatic ligands), while the downfield shifts are caused by σ-contact mechanisms. (c) Dipolar Shifts. The predominantly downfield shift for the equatorial ligands and the shift in the opposite direction for axial ligands suggest that these shifts may be dipolar. Consider the simplified equation for an axially symmetric system,26,27 given in eq 3.31 The final term, the geometric factor, shows

δdipolar )

-β2S(S + 1) 2 (1 - 3 cos2 θ) (g| - g⊥2) 9kT r3

(3)

the distance and angular dependence of the dipolar shift, r being the distance from the paramagnetic center to the NMR nucleus and θ being the angle relative to the z axis of the molecule. The expression (1 - 3 cos2 θ) defines a cone (θ ) 54.7°) that is aligned along the z axis of the molecule as shown in IV.

2-MeTHF

o-H o-H m-CH3 o-H m-H p-H o-H m-H p-CH3 2-H 3-H 2-CH3 2-H 3/4-H

5-H

(31) As reported in ref 26, the equation predicts the incorrect sign for Fe(III) porphyrin systems (see Supporting Information). The omission of the minus sign has been corrected in the equation reported here. (32) Collman, J. P.; Barnes, C. E.; Swepston, P. N.; Ibers, J. A. J. Am. Chem. Soc. 1984, 106, 3500.

[1a(L)2]+[BF4]-

-51 -1.3 -49 -47 -1.7 -33 -47 -1.8 17 -19 -15 -17.8 -16.8 -14.4 -14.6 -15.2 -15.5 -18.4

-90 -16 -83 -89 -16 -45 -73b -9b 20b -29 -22 -32.0c -33.2c -24.6c -25.5c -29.4c -29.6c -27.3c

At -58 °C except where noted. b At -38 °C. c Tentative assignments. a

Any axial ligands lie inside the cone, where the geometric factor is negative. Thus, the dipolar shifts will be in opposite directions for the equatorial and axial ligands. Dipolar shifts can be calculated when both g values and structural information are available. Neither are available for the diruthenium tetraoctanoates, but using the g values for other derivatives, the signs of the dipolar shifts can be determined. For Ru2(O2CR)4, where R ) (CH2)8CH3, magnetic susceptibility measurements gave g| ) 1.93 and g⊥ ) 2.22.18 For [Ru2(O2CR)4]+[Cl]-, where R ) (CH2)2CH3, EPR spectra gave g| ) 1.947 and g⊥ ) 2.200.16 Thus, both neutral and cationic diruthenium tetracarboxylate species have negative (g|2 - g⊥2) values. For axial ligands, where θ is less than 54.7°, a negative dipolar (upfield) shift is expected, and for equatorial ligands, a positive (downfield) shift is expected.22 Since the isotropic shifts seen for 1a correspond to this, the dipolar mechanism appears to be a major contributor. The 1H NMR signals for the axial ligands (Table 2) are all seen upfield, while the majority of the protons on the equatorial octanoate ligands are shifted downfield. The exception is CH2(2), whose isotropic shift has significant contact shift contributions. (d) Contact Shifts. The contact shift depends on bonding interactions, which influence the sign and magnitude of Acon, the hyperfine coupling contant for a nucleus and an unpaired electron (eq 4).26 The effect of an unpaired electron is transmit

δcontact ) With monomeric axially-symmetric systems, the center of the cone is placed on the paramagnetic metal center. In dinuclear systems where the metal centers are bound directly to each other, the electron density is distributed over the M-M bond and the origin of the cone may be placed in the center of the M-M bond, as in IV. Studies on Ru24+ bis(porphyrin) systems32 have shown this approach to be more valid than placing a cone at each metal center. The location of a nucleus with respect to the cone will influence its dipolar shift. At the surface of the cone, the geometric factor goes to zero. The equatorial ligands of Ru2(O2CR)4 lie outside the cone, where the factor is positive.

1a(L)2

AcongjβS(S + 1) 3gnpkT

(4)

ted through molecular orbitals. The transfer may occur via direct delocalization, where unpaired electron density is transferred from the metal orbitals containing the unpaired electrons directly into empty ligand orbitals, or via indirect spin polarization, where unpaired spin density is transferred to fully-occupied metal orbitals and then to ligand orbitals, or both. Both the delocalization and polarization mechanisms may occur through σ and π orbitals. In the Ru2(O2CR)4n+ species studied here, π-contributions to the contact shift were expected to dominate since the unpaired electrons occupy π* (and δ*, in the case of [1a]+[X]-) orbitals. The π-type mechanisms were investigated using aromatic and unsaturated carboxylates and π- and σ-axial ligands, as discussed below. Contributions from σ-polarization mechanisms could occur as well. For the equatorial ligands, such contributions are insignificant. The isotropic shifts of the octanoate 13C NMR signals (Table 3) did not show the

3648 Inorganic Chemistry, Vol. 35, No. 12, 1996 Table 3.

13C

NMR Assignmentsa Based on T1 Values for 1a THF-d8

δobs (ppm)

δiso (ppm)

toluene-d8 δobs (ppm)

T1 (s)

C(1) -92b -277 C(2) -72b -110 C(3) 13.0 -14.7 1.05 ( 0.06 C(4) 30.0 -0.1 1.64 ( 0.03 C(5) 30.3 0.3 2.81 ( 0.06 C(6) 32.4 -0.4 4.14 ( 0.08 C(7) 22.7 -0.8 5.28 ( 0.14 C(8) 13.1 -1.2 6.3 ( 0.2 a

Chisholm et al.

11.3 27.3 29.5 32.4 23.3 14.4

δiso (ppm)

T1 (s)

-16.0 0.37 ( 0.04 -2.4 0.66 ( 0.06 -0.1 1.23 ( 0.02 0.0 1.80 ( 0.07 0.2 3.00 ( 0.08 0.2 4.10 ( 0.07

At 61 °C. b Tentative assignments.

alternating pattern typically caused by σ-polarization.33 Analysis of the chemical shifts for axial ligands (see below) indicates that σ-contact mechanisms are occurring, although π-contributions are larger. (e) 1/T Dependence and Zero-Field Splitting Contributions to the Dipolar Shift. Equations 3 and 4 both predict that the 1/T dependence of the chemical shift will be linear and that the observed chemical shift will approach the diamagnetic value as 1/T approaches zero. However, deviation from Curie behavior is seen when low-lying states are populated,34 the hyperfine coupling is temperature dependent,35 or large zerofield splitting occurs.36 Of these, the last should be most significant for the diruthenium systems described here. The effects of zero-field splitting on dipolar shifts are not accounted for in eq 3. In both 1a and [1a]+[X]-, zero-field splitting (ZFS) causes very large D values of ca. 290 cm-1 for 1a18 and ca. 75 cm-1 for [1a][X]-.16 (For mononuclear complexes, D values are usually less than 5 cm-1.37) The effect of ZFS on the dipolar shift is included in eq 5 (for S ) 1) and eq 6 (for S ) 3/2). The additional terms introduce a 1/T2

[

]

2β2(g|2 - g⊥2) 3 cos2 θ - 1 g|2 + 1/2g⊥2 D δdip ) 1(5) 9kT r3 3(g 2 - g 2) kT δdip )

[

⊥

|

]

5β2(g|2 - g⊥2) 3 cos2 θ - 1 4(g|2 + 1/2g⊥2) D 1 (6) 12kT r3 5(g 2 - g 2) kT |

⊥

dependence that may be large enough to be seen at lower temperatures if the dipolar contribution dominates the isotropic shift. Since the (g|2 - g⊥2) term is negative for Ru2(O2CR)4n+, the ZFS contribution will enhance the dipolar shift; i.e., the downfield dipolar shifts seen for the equatorial ligands should be shifted even further downfield at lower temperatures. The VT 1H NMR data for 1a, [1a]+[BF4]-, and [1a]+[O2C(CH2)6CH3]- in the range -75 to +40 °C were plotted vs 1/T (Figures 5-7). For most signals, the temperature dependence is linear in the measured temperature range. However, for some (33) Morishima, I.; Okada, K.; Yonezawa, T.; Goto, K. J. Am. Chem. Soc. 1971, 93, 3922. (34) (a) La Mar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 63. (b) Bertini, I.; Luchinat, C.; Messori, L.; Vasˇa´k, M. J. Am. Chem. Soc. 1989, 111, 7300. (c) Banci, L.; Bertini, I.; Briganti, F.; Luchinat, C. New J. Chem. 1991, 15, 467. (35) (a) Wu¨thrich, K. Struct. Bonding 1970, 8, 53. (b) Horrocks, W. DeW., Jr.; Greenberg, E. S. Mol. Phys. 1974, 27, 993. (36) (a) Chmielewski, P. J.; Latos-Graz˘ yn´ski, L. Inorg. Chem. 1992, 31, 5231. (b) Latos-Graz˘ yn´ski, L. Inorg. Chem. 1985, 24, 1681. (c) Reference 34a. (d) Behere, D. V.; Birdy, R.; Mitra, S. Inorg. Chem. 1982, 21, 386. (37) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286. Please note that the signs of the equations taken from this article have been switched to correspond to the convention used here.

Figure 5. 1/T dependence of 1H NMR signals for 1a in THF-d8.

Figure 6. 1/T dependence of 1H NMR signals for [1a]+[BF4]- in THFd8.

signals, extrapolation of the data to 1/T ) 0, where both dipolar and contact contributions should become negligible, does not give the chemical shifts one would expect for the diamagnetic species. The protons most affected are the ones closest to the metal-metal bond: CH2(2), CH2(3), CH2(4) for 1a (Figure 5); CH2(3), CH2(4) for [1a]+[BF4]- (Figure 6) (note that CH2(2) is dominated by contact shift and thus not affected by ZFS); and CH2(2), CH2(3), CH2(4) for both equatorial and axial carboxylates in [1a]+[O2C(CH2)6CH3]- (Figure 7). Closer examination of some of the data shows nonlinear behavior that can be attributed to 1/T2 dependence. Figure 8 shows the data for CH2(2) of 1a. Although these protons have a large contact contribution (which causes the resonance to be shifted upfield relative to the others), the dipolar contribution is also expected to be strong due to the proximity of the protons to the metalmetal bond. In contrast, for [1a]+[BF4]- the CH2(2) data are linear. Here, the presence of an extra unpaired electron is expected to increase both dipolar and contact contributions due to the S(S + 1) factor in eqs 3 and 4. However, the cationic species have lower D values, which should decrease the dipolar contribution due to ZFS, and shorter Ru-O bonds, which should increase the through-bond contact interaction. Thus, CH2(2) is dominated by contact shift, causing linearity in 1/T and an intercept at 1/T ) 0, which corresponds to diamagnetic values. For CH2(3) and CH2(4), the relative dipolar contribution is larger, causing nonlinear behavior (Figure 9) and a negative chemical shift value at 1/T ) 0. For [1a]+[O2C(CH2)6CH3]-,

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3649

Figure 9. Non-Curie behavior of CH2(3) and CH2(4) of [1a]+[BF4]-. 1H NMR signals of CH (2) vary linearly with 1/T, presumably due to 2 large contact contributions.

Figure 7. 1/T dependence of 1H NMR signals for [1a]+[O2C(CH2)6CH3]in toluene-d8.

Figure 8. Non-Curie behavior of CH2(2) of 1a.

the curvature of the lines is not as apparent, but only six data points were taken over a smaller temperature range. (f) Axial Ligands. Addition of axial ligands to 1a in toluened8 results in an equilibrium between 1a(L)2 and [1a]n (eq 7).

[1a]n + 2L h 1a(L)2

(7)

For L ) pyridine and pyrazine, which coordinate more strongly than nitriles and THF, the equilibrium lies to the right, as indicated by room-temperature 1H NMR spectra which show the resolved signals of an axially-coordinated species. For L ) THF and benzonitrile, resolved signals are seen only at low temperature. At room temperature, the broad signals indicative of [1a]n are seen.

Axial ligand exchange also occurs when more than 2 equiv is present, preventing signals for the axially-coordinated ligands from being detected at room temperature. When the sample was cooled, the exchange process was slowed. The appearance of free and axially-coordinated ligands was generally seen by -18 °C, except for THF and benzonitrile, which needed to be cooled to -45 and -65 °C, respectively, suggesting that these ligands are not as strongly bound as the nitrogen heterocycles. Consistent with our earlier analysis, all the 1H NMR resonances for axial ligands of 1a(L)2 and [1a(L)2]+[X]- (Table 2) are shifted upfield due to the dipolar contributions expected for protons in close proximity to paramagnetic centers. A closer look at the overall shift patterns indicates that π-contact contributions are also occurring. The shifts for the ligands containing π-systems (pyrazines and pyridines) are significantly further upfield than for those without (THF).38 The effect seen for methyl substitution in the ligands with π-systems, where the CH3 resonance is paramagnetically shifted in the direction opposite to the H it replaced, also indicates that the contact contribution is due to π-interactions.27 Substitution of the 2-position in THF with a methyl group showed no such shifts for 1a and only slightly upfield shifts for [1a]+[X]-, confirming that no major π-contribution is occurring via a polarization mechanism. Further examination of the shift patterns indicates that direct π-delocalization of the unpaired electrons occupying the RuRu π* orbital into the aromatic π-system of the axial pyrazine and pyridine ligands is occurring. A nonattenuating upfield, downfield, upfield shift pattern for the ortho-, meta- and parapositions on an aromatic ring is indicative of π-delocalization.38 If the upfield dipolar shifts for the axial ligands pyrazine and pyridine are disregarded, such a pattern is seen. On the basis of similar chemical shift values, π-delocalizations are occurring to the same degree in both ligands. (g) 1H NMR Spectra of [1a]n. In regard to the above discussion, the broad signals seen for 1a in noncoordinating solvents can be explained. Half of the carboxylate ligands in the oligomeric species are involved with intermolecular -(- -[M2]- -O- -)n- interactions. These ligands are axial to one Ru24+ unit, as well as equatorial to another, resulting in the upfield axial shifts being counteracted by the downfield equatorial shifts. Since the oligomer is dynamic, breaking and reforming M- -O bonds either by rotation or by complete dissociation and association, exchange of the carboxylates in (38) Horrocks, W. DeW., Jr. In NMR of Paramagnetic Molecules: Principles and Applications; La Mar, G. N., Horrocks, W. DeW., Holm, R. H., Eds.; Academic Press: New York, 1973; Chapter 4.

3650 Inorganic Chemistry, Vol. 35, No. 12, 1996

Chisholm et al.

Table 4. Solvent Dependence of Electronic Spectra of 1a solvent

λmax (nm)

λmax (cm-1)

 (M-1 cm-1)

toluene benzene dichloromethane acetonitrile 1:2 benzene/pyrazine 1:2 benzene/pyridine

460 462 454 458 475 442 (sh) 406 438 438

21 740 21 650 22 030 21 840 21 060 22 620 24 630 22 840 22 840

1100 1200 710 830 3000 1490 1800 970 720

THF methanol

the axial site results in only one set of broadened signals. The butyrate derivative shows a very similar spectrum in noncoordinating solvents. Thus the broad signal at -10 ppm is assigned to CH2(2), while the signals for the remaining equatorial protons fall between -1 and +2 ppm. (h) UV-Visible Spectroscopic Studies. The solvent-dependent electronic absorption data for 1a in the visible region are given in Table 4. The characteristic absorption seen at ca. 460 nm ( ≈ 1100-1200 M-1 cm-1) in benzene and toluene solutions shifts to slightly higher energy with a change to oxygen-donor solvents, e.g. 438 nm in methanol and THF ( ≈ 720 and 970 M-1 cm-1, respectively). In the presence of pyrazine, there is a small but distinct red shift to 475 nm and, perhaps more significantly, the molar absorptivity increases to  ≈ 3000 M-1 cm-1. With pyridine (2 equiv) the axially-ligated complex 1a(py)2 shows a splitting of this absorption, namely λmax at 406 nm ( ≈ 1800 M-1 cm-1) and a shoulder at 442 nm ( ≈ 1490 M-1 cm-1). An unequivocal assignment of this band has never been made for Ru24+ tetracarboxylates. The molar absorptivity is consistent with a primarily M-M d-based electronic transition. In the symmetry point group D4h, assuming the 3A2g ground state of δ*2π*2, the Ru24+ π f π* (eu f eg) transition is allowed in the z direction. We believe the band observed at ca. 460 nm corresponds to this transition. Single-crystal polarization studies in the UV/visible region showed that the π f π* transition for the cationic Ru25+ species Ru2(O2CCH3)4Cl occurs at 460 nm ( ≈ 1000 M-1 cm-1),39 in good agreement with our assumption. Other possible assignments for the band seem unlikely. Of the δ/δ* f π* transitions, only δ* f π* (b1u f eg) is allowed by symmetry (x,y). Given the anticipated proximity of these orbitals, the energy of the observed transition (ca. 460 nm) seems too high for a δ* f π* transition. The δ* f π* transition for the cationic Ru25+ core has been seen at ca. 1450 nm (6900 M-1 cm-1) in the near-IR spectrum in single-crystal polarization studies.40 For the neutral compound 1a, we saw no evidence of a near-IR band; however, our data were obtained in solution. The π/π* f O2C π* transitions, being MLCT transitions, are also unlikely assignments for this absorption of relatively low intensity. For M2(O2CR)4 compounds where M ) Mo and W, the δ f O2C π* transitions have molar absorptivities of approximately 13 000 M-1 cm-1.41 Since these absorptions are at least 10 times as intense as the band seen for 1a, we feel comfortable in suggesting that the band at ca. 460 nm is indeed due to the π f π* transition. On the basis of this assignment, the shift in the electronic spectra and changes in molar absorptivity with different axial ligands must be due to π-interactions either with the Ru-Ru π* or Ru-Ru π orbitals. The π LUMOs of pyrazine, pyridine, (39) Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1988, 27, 2501. (40) Miskowski, V. M.; Loehr, T. M.; Gray, H. B. Inorg. Chem. 1987, 26, 1098. (41) Chisholm, M. H.; Clark, D. L.; Huffman, J. C.; Van Der Sluys, W. G.; Kober, E. M.; Lichtenberger, D. L.; Bursten, B. E. J. Am. Chem. Soc. 1987, 109, 6796.

Figure 10. Solution resonance Raman spectra of 1a showing the RuRu stretch in (a) benzene, (b) THF, (c) acetonitrile, and (d) benzene with 2 equiv of pyridine (asterisk ) solvent). Table 5. Solvent Dependence of the Ru-Ru Stretch of 1a in the Resonance Raman Spectra solvent

ν(Ru-Ru) (cm-1)

solvent

ν(Ru-Ru) (cm-1)

benzene THF

348 347

acetonitrile 1:2 benzene/pyridine

335 332

and acetonitrile are of correct symmetry to interact with the Ru-Ru π* HOMO, lowering the π* orbital in energy, resulting in a lower energy π f π* transition, and increasing the ligand character of the π* orbtial, resulting in a more allowed transition. This effect is most promiment for 1a(pz)2. Extended Hu¨ckel molecular orbital calculations show that the pyrazine LUMO, which is less than 1 eV higher in energy, does stabilize the Ru-Ru π* orbital,21 supporting this conclusion. Although the filled p orbitals of halides have been shown to interact with Ru-Ru π orbitals, increasing their energies and also decreasing the energies of the πfπ* transitions for [Ru2(O2C(CH2)2CH3)4]+[X]-,39 the ligands investigated here are not expected to have any significant interactions with the Ru-Ru π orbital. Methanol and THF, whose π lone pairs are directed away from the metal centers, are primarily σ-donors. The filled π orbitals of acetonitrile, pyridine, and pyrazine will lie at energies much lower than that of the Ru-Ru π* orbital, precluding strong interactions. Since the absorbances and molar absorptivities for [1a]n fall between those seen for 1a(L)2 with π-acceptor and non-π-acceptor ligands, we conclude that the axial carboxylates are also forming π-interactions with the Ru-Ru π* orbtials, although to a lesser degree. (i) Resonance Raman Spectroscopy. On the basis of the evidence for axial π-interactions in the visible spectra, we expected corresponding shifts in the Ru-Ru stretch, a transfer of Ru-Ru π* electrons to ligand π systems strengthening the Ru-Ru bond. Although the Ru-Ru stretch seen in resonance Raman spectra was affected by solvent (Figure 10, Table 5), the stretches for 1a(L)2 were lower for L ) pyridine and acetonitrile (332 and 335 cm-1 respectively) than for L ) THF (347 cm-1). Also [1a]n in benzene had a Ru-Ru stretch of 348 cm-1. The similarity of this stretch to that of 1a(THF)2, which has an axial Ru-O bond, supports the presence of axial -(- -[M2]- -O- -)n- interactions within [1a]n. The strength of the Ru-Ru bond is apparently affected much more by σ-interactions than π-interactions. This corresponds (42) Ketteringham, A. P.; Oldham, C. J. Chem. Soc., Dalton Trans. 1973, 1067.

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3651

Table 6. Cyclic Voltammetry of M2(O2C(CH2)6CH3)4 (M ) Ru, Rh, Mo)a,b E1/2(M24+/M25+) (mV)

Ru Rh Mo a

CH2Cl2

(CH2Cl2 CH3CN)

CH3CN

-233 820 152

(59) (84) (166)

-292 726 -14 nBu

(CH3CN THF)

THF

(77)

-369

(66)

-80

b

Electrolyte solutions contained 0.1 M 4NPF6. All E1/2 values referenced to the ferrocene/ferrocenium couple in the appropriate solvent.

to Raman studies done on Rh2(O2CCH3)4,42 which showed that the Rh-Rh stretch is influenced by axial interactions with the σ* orbital, decreasing with increasing donor strength. Unfortunately, nitrogen ligands were not included in this study, and thus, direct comparisons cannot be made to our results. For 1a in oxygen-donor and noncoordinating solvents, where carboxylate oxygens are presumed to occupy the axial site, the Ru-Ru stretch was 348 cm-1. In the presence of nitrogen-donors, the stretch was lowered to 335 cm-1. Similar behavior was seen for Mo2(O2C(CH2)6CH3)4 (405 cm-1 in THF, 397 cm-1 in acetonitrile). Contrary to what one might expect, the Ru-Ru stretches in the neutral species, with bond orders of 2, were slightly higher than those previously measured for the cationic species, with bond orders of 2.5. In the solid state, Ru-Ru stretches reported for Ru2(O2CR)4Cl (R ) H, CH3, C2H5, C3H7) range from 326 to 331 cm-1.43 In ethanol, where the axial chlorides are replaced by solvent, Ru2(O2CC3H7)4+ has a Ru-Ru stretch of 341 cm-1.43a The similar stretches of Ru2(O2CR)4 and Ru2(O2CR)4+ species support the ground state configuration of δ*2π*2 for the former. The δ*1π*3 state, with increased occupation of the π* orbital, would lead to a lower value for ν(Ru-Ru). (j) Electrochemical Studies. The effect of the solvent, donor vs nondonor, on the 1a/[1a]+ redox couple reflects the energy of the HOMO of 1a as a function of axial ligation. It is, of course, also a measure of the relative stability of the cationic species and its solvation. In order to separate these two factors, or at least evaluate them in a comparative fashion, we measured the corresponding redox couple for Mo2(O2C(CH2)6CH3)4 in the same solvents. We also attempted to obtain the data for Rh2(O2C(CH2)6CH3)4 and its cation, but due to its high redox potential, we were limited to CH2Cl2 and CH3CN as solvents. The data are collected in Table 6. The first point to note from the data is that the ease of oxidation of M2(O2C(CH2)6CH3)4 compounds follows the order M ) Ru > Mo > Rh. Since these are all second-row transition elements, this is a good indication of the M-M HOMO energy. Thus, it is easier to oxidize the Ru24+ complexes with the M-M electronic configuration σ2π4δ2δ*2π*2 (where electrons occupy higher energy antibonding orbitals) relative to the Mo24+ complexes with the M-M configuration σ2π4δ2. The large difference between the oxidation potentials of the Ru2 and Rh2 complexes is most striking, especially since the M-M HOMO for each is π*2 and π*4, respectively. We suggest that three factors are responsible for this: (1) Rh is more electronegative than Ru.44 (2) Oxidation of the Ru24+ core yields Ru25+ with three unpaired electrons (43) (a) Clark, R. J. H.; Franks, M. L. J. Chem. Soc., Dalton Trans. 1976, 1825. (b) Clark, R. J. H.; Ferris, L. T. H. Inorg. Chem. 1981, 20, 2759. (c) Miskowski, V. M.; Loehr, T. M.; Gray, H. B. Inorg. Chem. 1988, 27, 4708. (44) Pauling electronegativities: Rh, 2.28; Ru, 2.2; Mo(II), 2.16. From: Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and ReactiVity, 4th ed.; Harper Collins: New York, 1993.

whereas the related oxidation of the Rh24+ center yields only one unpaired electron.45 Thus, the former is stabilized to a maximum degree by Hund’s rule, which favors a ground state having the highest spin multiplicity. (3) It is known that, with strong axial ligands, the Rh2(O2CR)4+ has a HOMO with one unpaired electron in a M-M orbital of σ character.46 Thus, the simple d-orbital splitting pattern shown in II is beginning to break down for the d7-d7 tetracarboxylates of rhodium. The data in Table 6 also show the influence of solvent. The trend THF > CH3CN > CH2Cl2 indicates that the ease of oxidation increases with the ability of the solvent to stabilize the cation. An interesting comparison emerges between the M24+/M25+ couples (M ) Ru, Mo) in the solvents CH2Cl2 and CH3CN. The influence of solvent change for M ) Ru is modest (59 mV) compared to that for M ) Mo (166 mV). This we believe is due to the fact that Ru2(O2C(CH2)6CH3)4 is in fact an oligomer in CH2Cl2 (as noted earlier), whereas the Mo24+ carboxylate binds axial ligands much more weakly. (This matter has been discussed in terms of the temperature range of the mesophases for the M2(O2C(CH2)6CH3)4 compounds.3a) The Arenecarboxylates. In the case of Mo2(O2CAr)4 complexes, there is a significant interaction between the M-M δ HOMO and the aromatic ring via the O2C π system. This interaction occurs due to the favored coplanar arrangement for the O2CR and aromatic π systems and manifests itself in a strong red shift in the δ f O2C π* MLCT absorption.47 In the case of Ru2(O2CAr)4, charge transfer bands involving the carboxylates are not seen. Thus, in order to investigate the interactions between the Ru2n+ (n ) 4, 5) core and the aromatic rings, we have carried out certain structural studies along with an investigation of the way in which the paramagnetic Ru2n+ (n ) 4, 5) cores influence the 1H NMR signals within the aromatic ring. The 1H NMR data for the arenecarboxylates, as discussed below, indicate that unpaired spin density is being transferred to the aromatic π orbitals. These studies were meant to determine which molecular orbitals and, thus, which contact mechanisms might be responsible for the isotropic shifts of the equatorial carboxylates. A direct π-delocalization mechanism would involve transfer from the π* orbtial as shown in V. For

the aromatic π system to overlap with the carboxylate orbital, the ring would need to be perpendicular to the plane of the carboxylate. A π-polarization mechanism would involve the planar O2C-aromatic ring arrangement seen for Mo2(O2CAr)4 but would require that the unpaired electrons be polarizing the (45) Kawamura, T.; Katayama, H.; Nishikawa, H.; Yamabe, T. J. Am. Chem. Soc. 1989, 111, 8156. (46) (a) Drago, R. S.; Cosmano, R.; Telser, J. Inorg. Chem. 1984, 23, 3120. (b) Kawamura, T.; Fukamachi, K.; Sowa, T.; Hayashida, S.; Yonezawa, T. J. Am. Chem. Soc. 1981, 103, 364. (c) Kawamura, T.; Fukamachi, K.; Hayashida, S. J. Chem. Soc., Chem. Commun. 1979, 945. (47) San Filippo, J., Jr.; Sniadoch, H. J. Inorg. Chem. 1976, 15, 2209. (48) (a) M ) Cr, R ) Ph, ∠ ) 6.0°: Cotton, F. A.; Extine, M. W.; Rice, G. W. Inorg. Chem. 1978, 17, 176. (b) M ) Mo, R ) Ph, ∠ ) 7.2°: Collins, D. M.; Cotton, F. A.; Murillo, C. A. Inorg. Chem. 1976, 15, 2950. (c) M ) W, R ) Ph, ∠ ) 10.7°: Cotton, F. A.; Wang, W. Inorg. Chem. 1984, 23, 1604. (d) M ) Co, R ) Ph, ∠ ) 5.9°: Davies, J. E.; Rivera, A. V.; Sheldrick, G. M. Acta Crystallogr. 1977, B33, 156. (e) M ) Rh, R ) Ph, ∠ ) 1.2°: Simmons, C. J.; Clearfield, A.; Sun, Y. Inorg. Chim. Acta 1986, 121, L3. (f) M ) Cu, R ) Ph, ∠ ) 4.5°: Speier, G.; Fu¨lo¨p, V. J. Chem. Soc., Dalton Trans. 1989, 2331.

3652 Inorganic Chemistry, Vol. 35, No. 12, 1996

Chisholm et al.

Table 7. Crystallographic Data for 1c(THF)2‚2THF, [1c(THF)2]+[BF4]-, and 1c(CH3CN)2‚3CH3CN formula color crystal dimens (mm) space group T (°C) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) Z (molecules/cell) V (Å3) calcd density (g/cm3) wavelength (Å) mol wt linear abs coeff (cm-1) detector to sample dist (cm) sample to source dist (cm) scan speed (deg/min) scan width (deg + dispersion) individual background (s) 2θ range (deg) tot. no. of reflns collected no. of unique intensities no. with F > 0.0 no. with F > 3.0σ(F) no. with F > 2.33σ(F) R for averaging R(F) Rw(F) goodness of Fit for the last cycle max ∆/σ for last cycle a

1c(THF)2‚2THF

[1c(THF)2]+[BF4]-

1c(CH3CN)2‚3CH3CN

Ru2C48H60O12 deep red 0.16 × 0.32 × 0.40 P1h -154 10.730(5) Å 12.335(6) 9.193(4) 105.15(2) 109.35(2) 77.98(2) 2a 1098.20 1.559 0.710 69 515.57a 7.353 22.5 23.5 4.0 2.5 6 6-45 5931 2892 2804 2657

Ru2C40H42O10BF4 orange 0.25 × 0.25 × 0.35 C2/m -172 13.056(4) Å 21.358(6) 9.199(2)

Ru2C42H43N5O8 red 0.20 × 0.36 × 0.36 C2/c -169 27.058(3) 10.049(1) 17.956(2)

111.28(1)

120.89(1)

2 2390.10 1.350 0.710 69 971.71 6.798 22.5 23.5 10.0 2.0 4 6-45 2005 1605 1522

4 4190.04 1.465 0.710 69 923.95 7.588 22.5 23.5 8.0 2.0 4 6-55 6504 4826 4711 4535

0.024 0.0246 0.0255 1.002 0.001 for non-H 0.26 for H

1329 0.013 0.0395 0.0431 1.436 0.11

0.039 0.0698 0.0788 2.520 0.005

For Z ) 2, the asymmetric unit contains RuC24H30O6.

electron spin density in the δ orbital (VI). In the previously structured M2(O2CAr)4 compounds,48,49 the orientation of the ring depends on electronics and sterics, the former favoring a planar and the latter a perpendicular arrangement. In Ru2(O2CAr)4 complexes however, the molecular orbital V may shift the balance toward a perpendicular arrangement, without introducing ortho-substituents. The X-ray structures reported here show that the aromatic rings are essentially parallel to the carboxylate planes. This corresponds to structures already reported for Ru2(O2CAr)4n+ systems where Ar ) C6H5 and n ) 0, 16b and Ar ) p-C6H4OMe and n ) 1,6a suggesting that the solid-state orientation is not significantly affected by the aryl group, the axial ligand, or charge and that the electronic contributions still favor the parallel arrangement. (a) X-ray Structures of Ru2(O2C-p-C6H4CH3)4(THF)2 (1c(THF)2) and [Ru2(O2C-p-C6H4CH3)4(THF)2]+[BF4]- ([1c(THF)2]+[BF4]-). Up to this point, comparison of Ru2(O2CR)4n+ (n ) 0, 1) species has been limited to species containing

Table 8. Selected Bond Distances (Å) and Bond Angles (deg) for 1c(THF)2‚2THF

(49) (a) M ) Cr, R ) 2-C6H5Ph, ∠ ) 45°: Cotton, F. A.; Thompson, J. L. Inorg. Chem. 1981, 20, 1292. (b) M ) Mo, R ) 2-C6H5Ph, ∠ ) 48°: Cotton, F. A.; Thompson, J. L. Inorg. Chem. 1981, 20, 3887. (c) M ) W, R ) 2,4,6-Me3Ph, ∠ ) 42°: ref 48c. (d) M ) Rh, R ) 2-PhC6H5, ∠ ) 74°: Cotton, F. A.; Thompson, J. L. Inorg. Chim. Acta 1984, 81, 193. (e) M ) Cu, R ) 2,6-MeO-Ph, ∠ ) 58°: Erre, L. S.; Micera, G.; Piu, P.; Cariati, F.; Ciani, G. Inorg. Chem. 1985, 24, 2297. (50) As noted in the Experimental Section, the full-matrix least-squares calculations of [1c(THF)2]+[BF4]- did not allow the exact [1c]+:[BF4]ratio to be determined. However, on the basis of the method of preparation and IR and NMR characterization, a 1:1 ratio was assumed. Comparison of the structural data with those for 1c(THF)2 also gave no indication that both cationic and neutral Ru2(O2CR)4 units were present in the crystal.

Angles

Distances Ru(1)-Ru(1)′ Ru(1)-O(2) Ru(1)-O(4) Ru(1)-O(12) Ru(1)-O(14) Ru(1)-O(22)

2.2689(11) 2.0702(22) 2.0570(22) 2.0578(22) 2.0692(22) 2.3696(24)

O(2)-C(3) O(4)-C(3) O(12)-C(13) O(14)-C(13) C(3)-C(5) C(13)-C(15)

1.266(4) 1.276(4) 1.266(4) 1.275(4) 1.489(4) 1.482(4)

Angles Ru(1)′-Ru(1)-O(2) 90.47(7) Ru(1)′-Ru(1)-O(14) 89.55(7) Ru(1)′-Ru(1)-O(4) 88.83(7) Ru(1)′-Ru(1)-O(22) 173.59(5) Ru(1)′-Ru(1)-O(12) 89.75(7) Table 9. Selected Bond Distances (Å) and Bond Angles (deg) for [1c(THF)2]+[BF4]Distances Ru(1)-Ru(1)′ Ru(1)-O(2) Ru(1)-O(4) Ru(1)-O(12)

2.2618(16) 2.013(3) 2.107(4) 2.258(6)

O(2)-C(3) O(4)-C(3) C(3)-C(5)

1.274(6) 1.271(6) 1.478(8)

Ru(1)-Ru(1)′-O(2) 89.69(12) Ru(1)-Ru(1)′-O(12) 178.025(16) Ru(1)-Ru(1)′-O(4) 89.54(11)

not only different charges but also different axial or equatorial ligands. Here we report the structures of neutral and cationic diruthenium tetracarboxylates which contain the same carboxylate groups and axial ligands. Crystallographic data are given in Table 7, selected bond distances and angles are presented in Tables 8 and 9, and ORTEP drawings are shown in Figures 11 and 12.50

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3653 Table 10. Selected Bond Distances (Å) and Bond Angles (deg) for 1c(CH3CN)2‚3CH3CN Distances Ru(1)-Ru(1)′ Ru(1)-O(2) Ru(1)-O(4)′ Ru(1)-O(12) Ru(1)-O(14)′ Ru(1)-N(22) O(2)-C(3)

2.2757(10) 2.061(4) 2.066(4) 2.056(4) 2.067(4) 2.331(5) 1.260(7)

O(4)-C(3) O(12)-C(13) O(14)-C(13) O(14)′-C(13)′ N(22)-C(23) C(23)-C(24)

1.288(7) 1.270(8) 1.280(7) 1.280(7) 1.144(9) 1.454(9)

Angles Ru(1)′-Ru(1)-O(2) Ru(1)′-Ru(1)-O(4) Ru(1)′-Ru(1)-O(12) Ru(1)′-Ru(1)-O(14)

Figure 11. ORTEP drawing of Ru2(O2C-p-C6H4CH3)4(THF)2.

Figure 12. ORTEP drawing of [Ru2(O2C-p-C6H4CH3)4(THF)2]+[BF4]showing only one set of partially-occupied THF sites.

The primary difference between the two species, the charge of the core, did not affect the Ru-Ru bond distance. A distance of 2.2689(11) Å for 1c(THF)2 with four antibonding electrons compared with a distance of 2.2618(16) Å for [1c(THF)2]+[BF4]with only three antibonding electrons supported the conclusion made by Cotton et al.5d that the additional electron in Ru2(O2CR)4 must occupy the δ*, rather than the π*, orbital. Increased occupation of the π* orbÄ ital, which is more antibonding, to give the electron configuration δ*1π*3 would have significantly lengthened the Ru-Ru bond. The identity of the axial ligand in the cationic species does affect the Ru-Ru bond, however. The Ru-Ru distance in [1c(THF)2]+[BF4]- is comparable to the Ru-Ru distance of 2.265(2) Å in Ru2(O2CC6H5)4(C2H5OH)2+.4h Both distances are

89.58(12) Ru(1)′-Ru(1)-N(22) 172.70(15) 89.63(12) Ru(1)-N(22)-C(23) 155.8(5) 89.08(12) N(22)-C(23)-C(24) 179.0(7) 90.19(11)

ca. 0.015 Å longer than the Ru-Ru distance of 2.248(1) Å in Ru2(O2CCH3)4(H2O)2+ 4b and are up to 0.03 Å shorter than the Ru-Ru distances (2.267-2.290 Å) in the reported Ru2(O2CR)4+ species with Cl- coordinated in the axial position.4b,h,i Both the Ru-O and Ru-O(THF) distances in [1c(THF)2]+[BF4]- (2.015 (average) and 2.258(6) Å, respectively) were shorter than those for 1c(THF)2 (2.064 (average) and 2.3696(24) Å, respectively). This was attributed to the contraction of the Ru d orbitals with the increased charge. Similar Ru-O distances are reported for other structurally-characterized Ru2(O2CR)4+ cores.4b,h,i The dihedral angles between the carboxylate plane and the aromatic ring do vary between the neutral and cationic compounds, but both are essentially planar. For 1c(THF)2, the angles are 16.6 and 13.8°. For [1c(THF)2]+[BF4]-, they are 7.3°. This difference can be simply attributed to lattice packing forces. Recall that the lattice of [1c(THF)2]+[BF4]- contains the BF4- counterion, while the lattice of 1c(THF)2 contains an additional 2 equiv of THF. (b) X-ray Structure of Ru2(O2C-p-C6H4CH3)4(CH3CN)2 (1c(CH3CN)2). This structure, the first reported for a neutral diruthenium tetracarboxylate with nitrogen-containing axial ligands and potential π-acceptor ligands, contains a Ru24+ core essentially unaffected by the change in axial ligand. Crystallographic data are given in Table 7, selected bond distances and angles are presented in Table 10, and an ORTEP drawing is shown in Figure 13. Comparison to the bis-THF adduct (Table 8, Figure 11) shows that, despite enhanced axial ligand interactions, the Ru-Ru distance is insignificantly lengthened in the acetonitrile adduct. The Ru-N distance (2.331(5) Å) is shorter than the Ru-O(THF) distance (2.3696(24) Å). The Ru-Ru distance of 2.2757(10) Å is still within 3σ of that in the THF adduct (2.2689(11) Å). This suggests that no significant π-interactions occur. The Ru-Ru bond is not shortened, as would be expected should back-bonding from the Ru-Ru π* into the acetonitrile π* orbitals occur. Nor is the NtC distance (1.144(9) Å) lengthened from the NtC distance seen for the acetonitrile molecules in the lattice (1.142(12) Å). The axial acetonitrile ligands are linear (N-C-C is 179.0(7)°) but not collinear with the Ru-Ru bond (Ru-N-C is 155.8°). The angle is much larger than those seen for other end-bound nitriles (168.4-180°).51 (c) 1H NMR Spectra of Toluates and Benzoates. Although the solid state structures suggest that π-polarization must be occurring, free rotation of the aromatic rings in the arenecarboxylates in solution could still allow direct π-delocalization to occur. Therefore, the π-contributions to the isotropic shift of the equatorial carboxylates in Ru2(O2CAr)4n+ (Ar ) C6H5, (51) Storhoff, B. N.; Lewis, H. C., Jr. Coord. Chem. ReV. 1977, 23, 1.

3654 Inorganic Chemistry, Vol. 35, No. 12, 1996

Chisholm et al. Table 12. Estimated Dipolar Shift Contributionsa for p-Toluene Protons in 1c and [1c]+ with and without ZFS Terms δdip (ppm)

1c [1c]+

o-H m-H p-CH3 o-H m-H p-CH3

108r (cm)

θ (deg)

without ZFSb

with ZFSc

δiso (ppm)

4.813 6.921 8.819 4.756 7.074 8.734

67.06 74.59 86.08 64.20 71.92 85.73

2.14 1.04 0.63 1.76 0.88 0.65

8.56 4.16 2.51 8.45 4.23 3.10

9.2 1.3 0.6 24.5 -0.4 -5.6

a Calculated for 296 K. b Calculated using eq 3. c Calculated using eq 5 for 1c and eq 6 for [1c]+.

Table 13. Estimated Dipolar Shiftsa for Axial THF Ligands δdip (ppm)

1 [1]+

Figure 13. ORTEP drawing of Ru2(O2C-p-C6H4CH3)4(CH3CN)2. Table 11.

1H

NMR Data and Isotropic Shiftsa for Ru2(O2CR)4n+ Ru2(O2CR)4

Ru2(O2CR)4+

δobs (ppm) δiso (ppm) δobs (ppm) δiso (ppm) b: R ) C6H5 o-H m-H p-H c: R ) p-C6H4CH3 o-H m-H p-CH3 d: R ) (CH2)2CH3 CH2(2) CH2(3) CH3(4) e: R ) (CH)2CH3 CH(2) CH(3) CH3(trans) f: R ) (CH)C(CH3)2 CH(2) CH3(cis) CH3(trans)

16.5 9.3 10.7

9.1 1.1 3.3

31.8 7.5 19.7

24.4 -0.7 12.3

16.4 9.1 2.9

9.2 1.3 0.6

31.7 7.4 -3.3

24.5 -0.4 -5.6

2.9 7.4 6.3

0.1 5.6 5.4

-39 15.6 11.1

-42 13.8 10.2

7.0 16.3 1.9

0.5 9.1 -0.3

-10.9b 41.4b -15.0b

-17.4 34.2 -17.2

4.7 4.4 2.2

-1.4 2.2 0.2

-18.4c -5.4c -12.8c

-24.5 -7.6 -14.7

a Calculated using eq 1 and Mo (O CR) data. b T (ms): CH(2), 2 2 4 1 7.11 ( 0.12; CH(3), 7.06 ( 0.09; CH3(trans), 47 ( 1. c T1 (ms): CH(2), 6.6 ( 0.7; CH3(cis), 11 ( 1; CH3(trans), 44 ( 2.

p-C6H4CH3; n ) 0, 1), were investigated. The solubility of these compounds, even in coordinating solvents, was slight. 1H NMR spectra were taken in methanol-d4/THF-d8 mixtures, and the data are given in Table 11. Assignments of the proton signals were based on coupling patterns, relative intensities, and shifts upon methyl substitution. Isotropic shifts were calculated using the corresponding Mo2(O2CR)4 chemical shifts as the diamagnetic shifts. The solution isotropic shifts seen for the benzoate (1b) and toluate (1c) systems are not large, especially for the neutral systems, but do show an alternating downfield, upfield, downfield shift pattern for the ortho-, meta-, and para-protons, indicative of a π-contact mechanism.27,38 The upfield shift seen upon substitution of the para-proton with a methyl group confirms the π-nature of the contact contribution.27,38 For a system with π-delocalization, the shifts for the ortho-H and

2-H 3-H 2-H 3-H

108r (cm)

θ (deg)

without ZFSb

with ZFSc

δiso (ppm)d

4.430 5.655 4.304e 5.525e

24.07 27.72 24.07 27.72

-10.5 -4.55 -11.5 -4.87

-54.3 -23.5 -72.1 -30.7

-19 -15 -29 -22

a Calculated for 213 K. b Calculated using eq 3. c Calculated using eq 5 for 1 and eq 6 for [1]+. d From 1H NMR spectra of 1a and [1a]+[BF4]- and THF in toluene-d8 at -58 °C. e Calculated using the X-ray data from 1c, assuming a shortening of the Ru-O distance of 0.115 Å and no change in θ.

para-H should be comparable, which is not the case here (although the upfield dipolar contribution may diminish such an effect). Thus, the π-polarization mechanism appears to be occurring for the equatorial aromatic carboxylates, consistent with the planar arrangement seen in the solid state. (d) Estimated Dipolar Shifts. Using the data from the crystal structures of 1c(THF)2 and [1c(THF)2]+[BF4]-, the dipolar shift contributions for equatorial toluate and axial THF ligands were estimated. Comparison of the observed and calculated values, with and without zero-field splitting contributions, for the equatorial toluate protons (Table 12) should indicate the relative importance of ZFS effects and the degree of contact contribution. For 1c, the observed isotropic shift of the ortho-H, which is closest to the metal-metal bond, is in fairly good agreement with the value calculated with ZFS. The additional downfield shift can be attributed to π-contact contributions. For the meta-H and para-CH3, the observed isotropic shifts are actually closer to the value calculated without ZFS. This, however, is likely an effect of the π-contributions, which should cause upfield shifts at both positions, cancelling out the downfield dipolar shift. The calculations for the equatorial toluate protons in [1c(THF)2]+[BF4]- (S ) 3/2) indicated that its dipolar contributions were not significantly increased over those of 1c(THF)2 (S ) 1). When calculated using eq 3, which does not take ZFS into account, the dipolar shift values were actually slightly smaller due to the smaller (g|2 - g⊥2) term. The contribution from the ZFS term for [1c(THF)2]+[BF4]- is comparable to that for 1c(THF)2, despite the higher S value, since the D value for the cationic compound is lower (75 cm-1 compared to 290 cm-1 for S ) 1).16,18 The difference between the calculated and observed values is again attributed to π-contact contributions, which, as concluded from the VT 1H NMR data for 1a and [1a]+[X]-, effectively dominate in the S ) 3/2 systems. The X-ray data for the axial THF ligands in 1c(THF)2 were used to estimate its dipolar shift at -60 °C and compared to the 1H NMR data for 1a and [1a]+[BF4]- (Table 13). Here, it is apparent that ignoring ZFS does not predict sufficiently large

Ru2(O2CR)40,+ Complexes shifts. Inclusion of the ZFS term gives values significantly larger than the observed shifts for both 1a and [1a]+[BF4]- (by ∼40 ppm for 2-H and 8 ppm for 3-H), suggesting that the proton resonances for the axial ligands are shifted downfield by a contact mechanism. This is consistent with the conclusion made from the 1H NMR data for the axial carboxylate in [1a]+[O2C(CH2)6CH3]-. Since no evidence was seen for π-contributions with 2-MeTHF, a σ-contact mechanism must be occurring. Treating the system as two paramagnetic centers and calculating the dipolar shifts using the distance to the closest metal would only increase the dipolar estimate, confirming that placing the origin at the center of the metal-metal bond is a better model for the metal-metal multiply-bonded systems. The Unsaturated Carboxylates. Ru2(O2CR)4n+ systems with CdC bonds in conjugation with the carboxylate group confirmed that π-contact contributions occur in the equatorial ligands. Comparison of the 1H NMR chemical shifts of Ru2(O2C(CH)2CH3)4n+ (1e) and Ru2(O2C(CH)C(CH3)2)4n+ (1f), where n ) 0, 1 (Table 11), showed that substitution of the CH(3) proton with a methyl group affected all the proton signals, most likely due to geometry changes. Assignments were made using a combination of relative chemical shifts, intensity, coupling, T1 measurements and isotropic shifts calculated using Mo2(O2CR)4 chemical shifts. The upfield, downfield, upfield pattern seen for the crotonates is typical for π-contact contributions.52 Substitution of the CH(3) in Ru2(O2C(CH)2CH3)4+, which gave a signal downfield at 41.4 ppm, with a methyl group shifted the signal upfield to -5.4 ppm, also indicative of π-contributions. Further evidence for a contact mechanism is seen when the isotropic shifts of the saturated butyrate (1d) and unsaturated crotonate (1e) are compared. A large shift from -42 ppm for the sp3-hybridized CH2(2) to -17.4 ppm for the sp2-hybridized CH(2) occurs. The change in hybridization would not affect the dipolar shift significantly but would influence the hyperfine coupling (Acon), which determines the contact shift. Thus, evidence for contact shifts is seen for equatorial saturated, unsaturated, and arene carboxylates, the last two supporting a π-contact mechanism and the last, a π-polarization mechanism. Conclusions 1H NMR spectroscopy has proven to be a useful technique for studying paramagnetic Ru2(O2CR)4n+ species. Attempts to obtain a nonligated Ru2(O2CR)4 species in noncoordinating solvents were unsuccessful. 1H NMR, along with UV/visible, molecular weight, and electrochemical studies, indicated that 1a and its diamagnetic Rh24+ analogue are oligomeric species with extended π-systems in noncoordinating solvents. The axial geometry of the dimetal tetracarboxylate systems has allowed us to determine the factors contributing to the isotropic shifts of axial and equatorial ligands. The axial ligands are affected by both dipolar and contact shifts. Through-space interactions cause upfield shifts. One contact (through bonds) mechanism occurring with π-type ligands is direct π-delocalization, which suggests that Ru2(O2CR)4n+ units could be linked via their axial sites with bridging π-type ligands to form conductive polymers. Our investigation of this will be reported in a subsequent paper.21 Comparison of the estimated dipolar shifts for axial THF ligands with observed chemical shifts suggests that downfield σ-contact contributions occur as well. The equatorial carboxylate ligands are also affected by both dipolar and contact shifts. In the long-alkyl-chain carboxylates the downfield dipolar shift dominates. In aromatic and conju-

(52) Arafa, I. M.; Goff, H. M.; David, S. S.; Murch, B. P.; Que, L., Jr. Inorg. Chem. 1987, 26, 2779.

Inorganic Chemistry, Vol. 35, No. 12, 1996 3655 gated systems, evidence for π-contact shifts is seen, the contact shift providing a major component of the isotropic shifts of the cationic species. Temperature dependence studies and calculations of the dipolar shifts show that zero-field splitting contributes significantly to the dipolar shift. The similar 1H NMR spectra seen for bis-adducts of 1a with σ-donors (THF) and π-acceptors (pyrazine) and the similar RuRu bond distances seen for Ru2(O2C-p-tolyl)4L2, where L ) THF and CH3CN, suggest that the π-acceptor ligands do not change the δ*2π*2 ground state or significantly lower the δ*1π*3 state. The solvent dependences of the electronic and Raman spectra of 1a apparently reflect small electronic differences in π- and σ-interactions which combined have little effect on the Ru-Ru bond length. Experimental Section All ruthenium and molybdenum compounds were handled under argon or dinitrogen using Schlenk techniques and gloveboxes. Toluene, hexanes, THF, and diethyl ether were distilled from sodium or potassium with benzophenone, dichloromethane was distilled from calcium hydride under dinitrogen, and all were stored over molecule sieves. Methanol, acetonitrile, and diglyme were bought anhydrous from Aldrich and stored over molecular sieves. 1H NMR spectra were recorded on a Varian XL-300 spectrometer, UV/visible spectra on a HP8452A diode array spectrophotometer with UV/visible operating software 89531A, and IR spectra on a Nicolet 510P FT-IR spectrophotometer using KBr disks. Elemental analyses were performed by Oneida Research Services. Ru2(O2CCH3)4,5b Ru2(O2CH)4,5b Ru2(O2CCH3)4Cl,53 Rh2(O2CCH3)4,54 and Mo2(O2CR)455 were synthesized via literature methods. The carboxylic acids, pyrazine, 2,6-dimethylpyrazine, pyridine, 4-picoline, benzonitrile, CrCl2, and Mo(CO)6 were purchased from Aldrich and used without further purification. Synthesis of Ru2(O2CR)4 (1a-f). Method A. Toluene (200-250 mL) was added to Ru2(O2CCH3)4 or Ru2(O2CH)4 (0.7 mmol) and the appropriate carboxylic acid (3.5 mmol) in a Schlenk flask equipped with a distillation arm. The mixture was refluxed for several hours, and most of the solvent was distilled off, removing the toluene/acetic acid or toluene/formic acid azeotrope. For R ) (CH2)6CH3 (1a), the solution was filtered and the toluene removed in Vacuo. The brown residue was redissolved in methanol. 1a was isolated as a brown precipitate. Yield: 45% from Ru2(O2CH)4, 55-80% from Ru2(O2CCH3)4. Anal. Calcd for Ru2O8C32H60: C, 49.60; H, 7.80. Found: C, 49.55; H, 7.96. IR (cm-1): 2959 s, 2924 s, 2851 s, 1545 vs, 1522 m, 1468 m, 1454 m, 1435 s, 1412 s, 1317 w, 1180 w, 1111 w, 723 m, 675 m, 478 w. Subsequent batches of precipitate contained [1a]+[O2C(CH2)6CH3]-. Anal. Calcd for Ru2O10C40H75: C, 52.23; H, 8.23. Found: C, 52.67; H, 8.84. IR (cm-1): 2957 s, 2924 s, 2853 s, 1523 vs, 1493 s, 1456 vs, 1415 vs, 1317 w, 1107 m, 723 w, 667 m. For R ) (CH2)2CH3 (1d), the toluene solution was filtered and cooled to -20 °C. Precipitation generally yielded both 1d and [1d]+[O2C(CH2)2CH3]-. For R ) C6H5 (1b), C6H4CH3 (1c), (CH)2CH3 (1e), and (CH)C(CH3)2 (1f), the product, insoluble in toluene, was filtered off and washed with hexanes. 1b and 1c were recrystallized from THF; 1e and 1f, from methanol. IR for 1b (cm-1): 3094 w, 3065 w, 2964 m, 2878 w, 1595 m, 1549 s, 1495 w, 1175 w, 1157 w, 1070 m, 1042 m, 1028 m, 879 m, 845 m, 712 s, 689 m, 507 m. IR for 1c (cm-1): 3065 w, 3034 w, 2968 w, 2922 w, 2875 w, 1613 m, 1587 m, 1539 s, 1448 w, 1406 vs, 1293 w, 1178 m, 1143 w, 1020 m, 851 m, 838 m, 783 m, 754 s, 692 w, 636 m, 472 m. IR for 1e (cm-1): 3038 w, 2967 w, 2942 w, 2910 w, 2875 w, 2851 w, 1658 s, 1537 vs, 1498 m, 1445 m, 1415 vs, 1402 s, 1375 m, 1300 w, 1263 m, 1103 w, 966 m, 918 w, 841 w, 746 m, 688 w, 611 w, 488 w. IR for 1f (cm-1): 3034 w, 2970 w, 2930 w, (53) (a) Marchon, J.-C.; Maldivi, P. Private communication. (b) Mitchell, R. W.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1973, 846. (54) Rempel, G. A.; Legzdins, P.; Smith, H.; Wilkinson, G. Inorg. Synth. 1972, 13, 90. (55) Brignole, A. B.; Cotton, F. A. Inorg. Synth. 1972, 13, 81.

3656 Inorganic Chemistry, Vol. 35, No. 12, 1996 2912 w, 2856 w, 1647 s, 1528 vs, 1499 m, 1443 s, 1408 vs, 1369 m, 1313 s, 1186 m, 1076 m, 1023 m, 858 m, 763 m, 603 w, 495 w. Method B.14b Ru2(O2CR)4Cl (0.63 mmol) was dissolved in a 1:1 MeOH/H2O solvent mixture (20 mL). An aqueous solution of CrCl2 (0.95 mmol in ca. 10 mL) was added. For R ) (CH)C(CH3)2 (1f) and (CH2)2CH3 (1d), the product precipitated immediately and was isolated by filtration. IR for 1d (KBr, cm-1): 2963 s, 2934 m, 2874 m, 1550 vs, 1522 s, 1462 s, 1431 s, 1419 s, 1346 w, 1315 m, 1263 m, 1213 w, 1103 m, 1047 w, 897 w, 870 w, 800 m, 737 m, 652 m, 582 w, 451 m, 435 m. Method C. A Ru(II) blue solution56 (2.2 g, 8.4 mmol) in methanol (20 mL) was added to NaO2C(CH2)6CH3 (5.6 g, 33.7 mmol) in methanol (50 mL), and the mixture was refluxed for 22 h. The solution turned from blue to green to brown with the formation of precipitate. After cooling of the mixture to room temperature, an unidentified black precipitate was filtered off and washed with methanol. Yield: 1.09 g. IR (cm-1): 2957 s, 2924 s, 2855 s, 2811 m, 1936 w, 1732 w, 1522 vs, 1415 s, 1316 m, 1110 w, 1020 m, 551 m. 1H NMR (benzene-d6, δ): 0-2 ppm (vbr). UV/visible (benzene, λmax): 342 nm. Upon cooling of the mixture to 10 °C, 1a precipitated from the mother liquor. Yield: 1.56 g, 48%. Synthesis of Ru2(O2CR)4Cl ([1a-f]+[Cl]-). Method A. Ru2(O2CCH3)4Cl (1.6 mmol) was dissolved in hot methanol (40 mL). The appropriate carboxylic acid (7.2-8.0 mmol) was added and the solution stirred at 50 °C for several days. The solvent was removed and the residue checked by 1H NMR. If acetate groups were still present, more carboxylic acid and methanol were added and the solution was stirred longer. For R ) (CH2)6CH3 ([1a]+[Cl]-), the final residue was dissolved in toluene. Addition of hexanes caused precipitation of [1a]+[Cl]-. Yield: 93%. IR (cm-1): 2957 s, 2926 s, 2855 s, 1458 vs, 1431 vs, 1318 m, 725 w, 690 m, 679 m. For R ) (CH)2CH3 ([1e]+[Cl]-), the final residue was dissolved in THF. Addition of hexanes caused precipitation of [1e]+[Cl]-. IR (cm-1): 3047 w, 2969 w, 2942 w, 2913 w, 2855 w, 1653 s, 1416 vs, 1302 w, 1252 m, 1103 w, 1007 w, 965 m, 918 w, 839 w, 745 s, 696 w, 625 w, 509 m. Method B.53a Ru2(O2CCH3)4Cl (0.33 g) and HO2C(CH2)2CH3 (15 mL) were stirred at 170 °C. After all the solid had dissolved, the solution was cooled to 10 °C. The resulting precipitate was filtered off and washed with diethyl ether, yielding [1d]+[Cl]- (0.298 g). IR (cm-1): 2965 m, 2934 w, 2876 w, 1464 s, 1450 s, 1427 vs, 1329 m, 1265 m, 1211 m, 1097 m, 1020 w, 897 w, 808 m, 758 w, 736 w, 677 m, 459 m. Method C.57 Ru2(O2CCH3)4Cl (0.075 g, 0.158 mmol) was dissolved in 20 mL of a 1:1 MeOH/H2O mixture. The carboxylic acid (0.95 mmol) dissolved in methanol was added to the Ru2(O2CCH3)4Cl solution. The reaction mixture was stirred at 90-95 °C for 21/2 h. The precipitate was filtered off and washed with diethyl ether. IR for R ) C6H5 ([1b]+[Cl]-) (cm-1): 3068 w, 3054 w, 1601 m, 1497 m, 1466 s, 1408 vs, 1176 w, 1026 m, 845 w, 716 m, 691 s, 530 m. IR for R ) C6H4CH3 ([1c]+[Cl]-) (cm-1): 3039 w, 3013 w, 2965 w, 2919 w, 1611 m, 1514 w, 1451 m, 1408 vs, 1180 m, 1020 w, 783 w, 758 s, 642 m, 500 m. Yield: 0.090 g, 73%. Synthesis of Ru2(O2CR)4BF4 ([1a-f]+[BF4]-). Method A.57 A solution of AgBF4 (0.14 mmol) in THF (15 mL) was added to Ru2(O2CR)4Cl (0.10 mmol) in THF (30 mL). The reaction mixture was stirred for 12 h and filtered over Celite. Addition of hexanes to the THF solution resulted in a precipitate. For R ) C6H5 ([1b]+[BF4]-) and C6H4CH3 ([1c]+[BF4]-), cooling the solution to -20 °C resulted in the formation of crystals. IR for [1b]+[BF4]- (KBr, cm-1): 3073 w, 2977 w, 2892 w, 1601 m, 1497 m, 1464 s, 1406 vs, 1181 w, 1143 w, 1057 s, 1024 s, 860 w, 845 w, 720 s, 690 s, 534 m. IR for [1c]+[BF4](cm-1): 2967 w, 2922 w, 2894 w, 1607 m, 1514 m, 1449 m, 1406 vs, 1182 s, 1145 w, 1057 s, 1020 m, 854 w, 783 w, 758 s, 642 m, 500 m, 480 m, 457 w. Method B. A solution of AgBF4 (0.14 mmol) in THF (15 mL) added to Ru2(O2CR)4 (0.10 mmol) and THF (30 mL) was worked up as in method A. (56) (a) Rose, D.; Wilkinson, G. J. Chem. Soc. A 1970, 1791. (b) Gilbert, J. D.; Rose, D.; Wilkinson, G. J. Chem. Soc. A 1970, 2765. (57) Barral, M. C.; Jime´nez-Aparicio, R.; Rial, C.; Royer, E.; Saucedo, M. J.; Urbanos, F. A. Polyhedron 1990, 9, 1723.

Chisholm et al. Synthesis of Rh2(O2C(CH2)6CH3)4 (3a). Method A. Rh2(O2CCH3)4MeOH2 was treated as in method A for Ru2(O2CR)4. The residue remaining after removal of toluene was redissolved in CH3CN. The first batches of blue precipitate contained what is believed to be Rh2(O2C(CH2)6CH3)5. 1H NMR (benzene-d6), δ): 17 (vbr), 4.8 (br), 2.2, 1.4, 0.9. 1H NMR (methanol-d4, δ): 10 (br), 2.9 (br), 0.9, 0.6. IR (KBr, cm-1): 2957 m, 2924 s, 2872 w, 2853 m, 1587 s, 1508 w, 1468 w, 1458 w, 1435 m, 1414 m. The subsequent batches of purple precipitate contained Rh2(O2C(CH2)6CH3)4(CH3CN)2. Drying in Vacuo at 45 °C produced Rh2(O2C(CH2)6CH3)4. 1H NMR (benzene-d6, δ): 2.9 (br), 20 (br), 1.39, 0.95. 1H NMR (methanol-d4, δ): 3.2 (mult, 2H, CH2(2)), 1.96 (t, 2H, CH2(3)), 1.32 (quint, 2H, (CH2(4)), 1.1 (mult, 4H, CH2(5), CH2(6)), 1.0 (mult, 2H, CH2(7)), 0.77 (t, 3H, CH3(8)). IR (cm-1): 2957 m, 2924 s, 2872 w, 2851 m, 1568 vs, 1522 w, 1468 w, 1433 m, 1414 s, 1313 w, 738 w, 679 m. Method B. A 25 mL portion of octanoic acid was added to Rh2(O2CCH3)4(MeOH)2 (0.27 g). The reaction mixture was stirred at 70 °C overnight. Precipitate formed after standing at room temperature several days. After filtration and washing with hexane, the product was recrystallized from acetonitrile. NMR Experiments. All 1H NMR spectra of the Ru2(O2CR)4n+ species were referenced to the protio solvent signal (benzene-d6, 7.15; toluene-d8, 2.09; methanol-d4, 4.78; THF-d8, 3.58 ppm). Spectra were accumulated using shortened delay times of 0.8 s for n ) 1 and 1.0 s for n ) 0. T1 times were measured by using the inversion recovery method and least-squares analysis available on the Varian XL-300. The available COSY and HETCOR routines were also used. Evans’ method experiments58 were performed on 10-3 M samples in 2% v/v tetramethylsilane solution. The toluene-d8 solutions of 1a gave µeff values of 2.84, 2.80, and 2.76 µB. THF-d8 solutions gave values of 2.69 and 2.86 µB. The 1H NMR resonances for axial ligands with π-systems (pyrazine, 2,6-dimethylpyrazine, pyridine, 4-picoline) and without (THF and 2-methyltetrahydrofuran) (Table 2) were located by adding 2-5 equiv and 10 equiv, respectively, of the ligand to 1a and [1a]+[BF4]- in toluene-d8. To slow any exchange processes and allow comparison of the temperature-dependent chemical shifts, samples were cooled to -58 °C, except for 4-picoline and [1a]+[BF4]-, which gelled at -40 °C. Interactions of benzonitrile with 1a were studied using 6 equiv of benzonitrile. The 13C NMR resonances for the carbons closest to the diruthenium core in 1a in both THF-d8 and toluene-d8 were broad and difficult to locate. Resonances in the 14-35 ppm range in THF-d8 were assigned using a 2-D HETCOR experiment. 13C NMR T1 measurements confirmed these assignments and were used to assign the signals for 1a in toluene-d8 (Table 3). 1H NMR Data for Mo (O CR) . For R ) C H (methanol-d /THF2 2 4 6 5 4 d8, δ): 7.4 (2H, o-H), 8.2 (2H, m-H), 7.4 (1H, p-H). For R ) p-C6H4CH3 (methanol-d4/THF-d8, δ): 7.2 (2H, o-H), 7.8 (2H, m-H), 2.3 (3H, p-CH3). For R ) (CH2)2CH3 (methanol-d4, δ): 2.8 (2H, CH2(2)), 1.8 (2H, CH2(3)), 0.9 (3H, CH3(4)). For R ) (CH)2CH3 (methanol-d4/ THF-d8, δ): 6.5 (1H, CH(2)), 7.2 (1H, CH(3)), 2.2 (3H, CH3(trans)). For R ) (CH)C(CH3)2 (methanol-d4/THF-d8, δ): 6.1 (1H, CH(2)), 2.2 (3H, CH3(cis)), 1.9 (3H, CH3(trans)). 13C NMR Data for Mo (O C(CH ) CH ) . In toluene-d at 61 °C 2 2 2 6 3 4 8 (δ, T1 (s)): 185.4 (C(1), 8.2 ( 0.3), 37.3 (C(2), 0.66 ( 0.04), 27.3 (C(3), 1.13 ( 0.03), 29.7 (C(4), 1.5 ( 0.1), 29.6 (C(5), 2.5 ( 0.1), 32.4 (C(6), 3.4 ( 0.1), 23.1 (C(7), 5.4 ( 0.4), 14.2 (C(8), 6.8 ( 0.3). In THF-d8 at 61 °C (δ, T1 (s)): 184.9 (C(1), 20 ( 1), 37.6 (C(2), 1.29 ( 0.03), 27.7 (C(3), 1.6 ( 0.1), 30.1 (C(4), 2.3 ( 0.1), 30.0 (C(5), 3.3 ( 0.1), 32.8 (C(6), 4.8 ( 0.1), 23.5 (C(7), 6.0 ( 0.2), 14.3 (C(8), 7.2 ( 0.3). Solution Molecular Weight Measurements. Measurements were performed under argon in freshly-distilled, degassed benzene using cryoscopic equipment assembled in-house. Solutions were cooled in an air-jacketed glass apparatus immersed in an ice bath. Changes in temperature were monitored using a thermistor which plotted resistivity on a chart recorder. The system was calibrated by using freshlysublimed biphenyl. (58) Evans, D. F. J. Chem. Soc. 1959, 2003.

Ru2(O2CR)40,+ Complexes

Inorganic Chemistry, Vol. 35, No. 12, 1996 3657

Table 14. Atomic Coordinates and Isotropic Thermal Parameters for Ru2(O2C-p-C6H4CH3)4(THF)2‚2THF

Table 15. Atomic Coordinates and Isotropic Thermal Parameters for Ru2(O2C-p-C6H4CH3)4(NCCH3)2‚3CH3CN

atom

104x

104y

104z

10Biso (Å2)

atom

104x

104y

104z

10Biso (Å2)

Ru(1) O(2) C(3) O(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) O(12) C(13) O(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) O(22) C(23) C(24) C(25) C(26) O(27) C(28) C(29) C(30) C(31)

692.9(2) 233(2) 2113(3) -951(2) 3263(3) 3077(3) 4139(3) 5416(3) 5587(3) 4538(3) 6558(4) 765(2) 64(3) 645(2) 70(3) -911(3) -909(3) 75(3) 1066(4) 1051(3) 80(4) 1958(2) 3250(3) 3121(4) 2196(4) 1299(4) 4138(3) 4033(4) 5541(4) 6270(4) 5387(14)

356.5(2) -480(2) -1075(3) 1200(2) -1657(3) -2473(3) -2997(3) -2701(3) -1875(3) -1370(3) -3271(3) 1724(2) 1798(3) -1048(2) 2825(3) 3127(3) 4107(3) 4808(3) 4479(3) 3524(3) 5876(3) 1076(2) 438(3) -137(3) 726(3) 1323(4) -5561(2) -6724(3) -5443(3) -6511(3) -7368(3)

1188.8(3) 454(2) -933(4) 1881(2) -1533(4) -2914(4) -3497(4) -2727(4) -1352(4) -750(4) -3360(5) 344(2) -1047(4) 1993(2) -1595(4) -2910(4) -3371(4) -2564(4) -1273(4) -783(4) -3047(5) 3798(2) 4345(4) 5519(5) 6310(5) 5029(4) -2353(3) -2472(5) -1807(5) -1267(5) -2397(5)

11 14 14 14 14 16 17 16 17 16 21 15 14 14 15 16 17 17 21 19 22 16 19 25 24 22 34 30 28 27 26

Ru(1) O(2) C(3) O(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) O(12) C(13) O(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) N(22) C(23) C(24) C(25) C(26) N(27) C(28) C(29)

4817.8(2) 4083(2) 5953(3) 5555(2) 6484(3) 6530(3) 7020(3) 7475(3) 7429(3) 6943(3) 8008(3) 5206(2) 5489(3) 4440(2) 5767(3) 5719(3) 5957(3) 6247(3) 6308(3) 6068(3) 6491(3) 4463(2) 4313(3) 4123(3) 3111(4) 3012(3) 2937(4) 4912(6) 4897(11)

673.1(5) 9525(4) 1472(6) 1820(4) 2319(6) 3431(7) 4239(7) 3912(7) 2773(6) 1987(6) 4762(7) 9491(4) 8474(6) 1860(4) 7635(6) 7959(6) 7177(6) 6010(7) 5698(6) 6499(6) 5075(7) 1826(5) 1951(6) 2136(7) 2240(8) 1583(7) 1071(9) 4900(16) 4650(25)

5310.8(3) 4770(3) 5680(4) 5833(3) 6064(4) 6536(4) 6894(4) 6771(4) 6293(4) 5948(4) 7148(5) 6399(3) 6412(4) 4215(3) 7208(4) 7924(4) 8644(4) 8687(4) 7982(4) 7253(4) 9467(5) 6065(4) 6551(4) 7169(5) 4109(5) 2748(5) 5250(6) 5323(9) 6039(16)

10 9 11 11 11 14 17 17 15 12 19 11 13 10 12 14 15 16 16 13 20 14 14 21 27 23 40 68(3) 48(5)

Cyclic Voltammetry. Cyclic voltammograms were run using an EG&G Model 273A potentiostat/galvanostat and Model 270 Electrochemical Analysis Software 3.00. A platinum disk working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl reference electrode were used. Sample solutions were ca. 1 × 10-3 M in 0.1 M nBu4NPF6 electrolyte. E1/2 values were referenced to the ferrocene/ferrocinium couple. Samples of M2(O2C(CH2)6CH3)4 scanned at 100 mV/s gave the following data. In CH2Cl2: M ) Ru, Ea - Ec ) 135.5 mV, E1/2 ) 192 mV, E1/2(cor) ) -233 mV; M ) Rh, Ea ) Ec ) 250 mV, E1/2 ) 1245 mV, E1/2(cor) ) 820 mV; M ) Mo, Ea - Ec ) 95.4 mV, E1/2 ) 552 mV, E1/2(cor) ) 152 mV. In THF: M ) Ru, Ea - Ec ) 97.5 mV, E1/2 ) 177 mV, E1/2(cor) ) -369 mV; M ) Mo, Ea - Ec ) 99.5 mV, E1/2 ) 466 mV, E1/2(cor) ) -80 mV. In CH3CN: M ) Ru, Ea - Ec ) 83.0 mV, E1/2 ) 94 mV, E1/2(cor) ) -292 mV; M ) Rh, Ea - Ec ) 164 mV, E1/2 ) 1122 mV, E1/2(cor) ) 736 mV; M ) Mo, Ea - Ec ) 103.5 mV, E1/2 ) 372 mV, E1/2(cor) ) -14 mV. Resonance Raman Studies. Excitation was provided by the 514.5 nm line of an Ar ion laser (Spectra Physics Model 2025). To minimize photodegradation, samples were spun at a rate of 50 rpm and laser power was kept below 100 mW. Scattered light was focused into a Jobin Yvon Mole S3000 triple monochromator (slits set to 3-4 cm-1 resolution). Samples were prepared in an inert-atmosphere glovebox and flame-sealed in 5 mm NMR tubes upon removal. X-ray Structure Determinations. General operating procedures and listings of programs used have been previously reported.59 A summary of the crystallographic data is given in Table 7. Data were collected using a standard moving-crystal, moving-detector technique. Data were corrected for Lorentz and polarization terms, and equivalent data were averaged. Structures were solved by direct methods (MULTAN78) and Fourier techniques. Atomic coordinates are given in Tables 14-16. (a) Ru2(O2C-p-C6H4CH3)4(THF)2‚2THF. Data were collected at -154 °C. Due to broad Ω scans, the scan width was chosen to be 2.5°. Unit cell dimensions were determined by a least-squares fit of (59) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg. Chem. 1984, 23, 1021.

Table 16. Atomic Coordinates and Isotropic Thermal Parameters for [Ru2(O2C-p-C6H4CH3)4(THF)2]+[BF4]atom

104x

104y

104z

10Biso (Å2)

Ru(1) O(2) C(3) O(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) O(12) C(13) C(14) C(15) C(16) O(17) C(18) C(19) B(20) F(21) F(22) F(23) F(24)

4380(1) 3514(3) 3868(4) 5260(3) 3225(5) 2207(5) 1602(6) 2013(7) 3044(7) 3642(5) 1371(7) 3185(5) 2553(14) 2959(11) 2152(13) 3343(14) 740(17) 0 314(16) 10000 9003(11) 9705(17) 9364(18) 10265(21)

5000 5666(2) 5873(2) 5670(2) 6366(2) 6546(3) 7009(3) 7296(3) 7118(3) 6655(3) 7795(3) 5000 4490(7) 5000 4598(7) 4587(8) 5000 4447(6) 5377(10) 3601(13) 3386(6) 4154(8) 3613(10) 3330(17)

8764(1) 9359(5) 10754(7) 8200(4) 11163(7) 10088(10) 10488(11) 11938(12) 12990(9) 12606(8) 12370(11) 6270(7) 5616(17) 3644(15) 3903(15) 5050(16) 9534(29) 10000 8764(26) 5000 5037(19) 4139(19) 3700(20) 4090(36)

29 34 34 34 38 56 65 63 59 49 72 41 70 39 93 71 105 93 90 46 94 146 153 249

the setting angles for 50 carefully centered reflections having 2θ values between 24 and 32°. No correction for absorption was carried out. Plots of four standard reflections measured every 300 reflections showed no significant trends. Following the initial refinement, all of the hydrogen atoms were located. The full-matrix least-squares refinement was completed by using anisotropic thermal parameters on the non-hydrogen atoms and isotropic thermal parameters on the hydrogen atoms. The final R(F) was 0.0246. All reflections were included. Reflections having F < 3.0σ(F) were given zero weight. The total number of variables was 401, including the scale factor and an overall isotropic extinction parameter. The final difference map was essentially featureless, the largest peak being 0.46 e/Å3 in the vicinity of the Ru atom and the largest hole being -0.35 e/Å3.

3658 Inorganic Chemistry, Vol. 35, No. 12, 1996 (b) [Ru2(O2C-p-C6H4CH3)4(THF)2]+[BF4]-. Data were collected at -172 °C. Unit cell dimensions were determined using 32 reflections. The ruthenium atom and toluate ligands were easily located, but the THF and BF4 moieties were badly disordered. In spite of the disorder, a difference Fourier map phased on the non-hydrogen atoms clearly located all hydrogen atoms (except those on the THF), and these were included in subsequent least-squares refinement. The exact stoichiometry of the sample could not be confirmed on the basis of crystallographic data, since the full-matrix least-squares refinement did not rule out partial occupancy of the BF4 site. A final difference Fourier map was essentially featureless, the largest peak being 0.21 e/Å3. (c) Ru2(O2C-p-C6H4CH3)4(CH3CN)2‚3CH3CN. Data were collected at -169 °C. Unit cell dimensions were determined by a leastsquares fit of the setting angles for 50 carefully centered reflections having 2θ values between 24 and 33°. No correction for absorption was carried out. Plots of four standard reflections measured every 300 reflections showed no significant trends. Following the initial refinement, many of the hydrogen atoms were evident in a differential Fourier map. Hydrogen atoms were introduced in fixed idealized positions and the full-matrix least-squares refinement was completed by using anisotropic thermal parameters on the nonhydrogen atoms. The final R(F) was 0.070 using the full unique data.

Chisholm et al. Reflections having F < 3.0σ(F) were given zero weight. The total number of variables was 253, including the scale factor and an overall isotropic extinction parameter. The final difference map was essentially featureless, the largest peak being 1.0 e/Å3 in the vicinity of C(28) (in a solvent molecule) and the largest hole being -1.1 e/Å3.

Acknowledgment. We thank the National Science Foundation for support of this work. We also thank Johnson Matthey for a generous loan of ruthenium trichloride, L. Zollars for assisting with the 2-D NMR experiments, Professor D. W. Margerum and L. Schurter for use of their near-IR spectrophotometer at Purdue University, and M. Wemple and H. Eppley for helpful discussions. Supporting Information Available: Complete listings of bond distances and angles, tables of anisotropic thermal parameters, and VERSORT drawings with complete atom number schemes for Ru(O2C-p-C6H4CH3)4(THF)2‚2THF, Ru2(O2C-p-C6H4CH3)4(CH3CN)2‚ 3CH3CN, and [Ru2(O2C-p-C6H4CH3)4(THF)2]+[BF4]- and calculations of dipolar shift sign changes (23 pages). Ordering information is given on any current masthead page. IC950860U